Can See Invisibility and Dispel Magic detect a target affected by Sequester?

Question

Sorry if it's dumb, but my reading of sequester (PHB, 274) suggests that it prevents any locate X spell (target) from detecting the target, as well as scrying (PHB, 273) and clairvoyance (PHB, 222) since these both create divination sensors, but wouldn't prevent the target from being visible to a creature using see invisibility (PHB, 274) since it doesn't target the hidden object/creature.

It might feel a bit weird, but remember it's only possible if you accurately deduce the room or location of the sequester'd item, being impossible to find it if someone cast sequester and hid it/never came back for it (so nobody could track it and deduce its location).

1. Can you use dispel magic (PHB, 234) and target "the closest object in range under the effect of sequester"? If not, what if you cast detect magic (PHB, 231) and then cast dispel magic targeting "the closest object affected by sequester which I'm sensing but not seeing"?

2. Could you see it with see invisibility (or truesight)?

3. Can you dispel it if you pour some flour on it? This would arguably get rid of the invisible condition.

4. More for curiosity purposes, can you sense it and then dispel it with detect thoughts (PHB, 231) if it's a creature (maybe by sensing its dreams)?

5. Finally, would nondetection prevent all that (except for the flour)?

I found a tangent question, but didn't see enough evidence of its answer, so I'm asking in here in full.

Answer 1

Yes, sequester can be somewhat defeated to varying degrees.

The description of sequester states (emphasis mine):

By means of this spell, a willing creature or an object can be hidden away, safe from detection for the duration. When you cast the spell and touch the target, it becomes invisible and can't be targeted by divination spells or perceived through scrying sensors created by divination spells.

Other effect details are explained, the object or creature is frozen in time until a condition is met, or:

This spell also ends if the target takes any damage.

(Note that while the description says the target can be hidden away, it does not state that the target has the hidden condition/state)

Seeing the Sequestered item

The invisible condition is defined in PHB p291:

An invisible creature is impossible to see without the aid of magic or a special sense.

As well as being able to detect via sound but it's in suspended animation, assumed silent and still.

See invisibilty specifically states:

For the duration, you see invisible creatures and objects as if they were visible

Which specifically allows you to visibily scan the area. But is that targetting the sequestered item with divination magic? No, see invisibility targets the person who was granted the ability to see invisibility. A DM may or may not agree mind you.

Either way, if the target has been removed from the area, or is not nearby while looking, it cannot be targeted by divination magic to locate it.

Detect magic is also a divination spell that would be targeting the one granted with magic detection.

you sense the presence of magic within 30 feet of you. If you sense magic in this way, you can use your action to see a faint aura around any visible creature or object in the area that bears magic

You would first need to be within 30 feet, allowing you to sense it, then use an action to see a faint aura, which only with see invisibility would the sequestered item be visible to you. You could sense it even if invisible, but no visibility without see invisiblity.

Truesight's description specifically states that you can see invisible objects and creatures. This comes with the boon of no divination magic being involved at all for the sake of DM disagreement.

Dispel Magic on Sequester

Dispell Magic is described as such:

Choose one creature, object, or magical effect within range. Any spell of 3rd level or lower on the target ends. For each spell of 4th level or higher on the target, make an ability check using your spellcasting ability. The DC equals 10 + the spell's level. On a successful check, the spell ends.

Sequester is a 7th level spell. So if you managed to find the object, you could target it (even if by feel, we don't need to roll to hit) with a DC of 17 to end the effect. Additionally, you can upcast Dispel Magic using a 7th level slot to auto dispel it:

When you cast this spell using a spell slot of 4th level or higher, you automatically end the effects of a spell on the target if the spell's level is equal to or less than the level of the spell slot you used.

Dispel Magic wouldn't be necessary

You could save your spell slots and just deal any amount of damage to the invisible object to end the effect per the description of Sequester. The difficulty though is that you need to be within about 30 feet or so to use any of these solutions so you'd need a way to locate the sequestered object from afar still.

Closer look at why Sequester is not the target of see invisible/detect magic

Scenario:

I am fighting a monster immune to attacks from non-magical weapons that cannot be targeted by spells, I cast magic weapon on my sword and then hit the monster, do I deal damage to it?

Is the monster now the target of my magic weapon spell? I don't think so, as per the spells' description I can't target the monster with "Magic weapon" nor can I target an (in)visible object with "see invisibility".

I would deal damage just as I would see the Sequestered item.

I do understand the confusion and "Target" does need tighter usage and clarification in general, and Sequester's spell description could explain that targets become Immune to Divination magic or effects but that's not what it says.

can be hidden away, safe from detection for...

This doesn't say "detection magic" or spells, nor does it literally mean it cannot be detected entirely (you can still detect by touch for example). It's fairly ambiguous as it is.

Answer 2

If you can find the sequestered creature, you can dispel the spell

Certain high level spells (e.g. Wall of Force) cannot be dispelled by Dispel Magic, but Sequester is not one of those. In fact, its duration is "Until Dispelled". Additionally, Dispel Magic says:

Choose one creature, object, or magical effect within range.

You don't actually need to see the target in order to cast the spell. As long as you know (or think you know) that a sequestered creature is in range, you can try to dispel it. Of course, you might not want to waste a spell slot on this, because:

This spell also ends if the target takes any damage.

So a simple unarmed strike will do the job just fine, as long as you're confident the creature will forgive you for the resulting bruise.

But can you find the creature? (Probably not)

So really, the hard part is finding the sequestered creature. The exact text of Sequester that talks about divination is:

By means of this spell, a willing creature or an object can be hidden away, safe from detection for the duration. When you cast the spell and touch the target, it becomes invisible and can't be targeted by divination spells or perceived through scrying sensors created by divination spells.

This is very similar to the text of Nondetection:

For the duration, you hide a target that you touch from divination magic. [...] The target can't be targeted by any divination magic or perceived through magical scrying sensors.

The wording isn't identical, but the only difference is that the protection of Sequester is limited to only spells, and not other kinds of magic that don't involve spellcasting. (I don't actually know if there are any non-spell divinations that could help someone located a sequestered creature. Perhaps some monsters have such abilities.) In any case, as long as we're talking about divination spells, the two wordings are exactly equivalent, which means that the logic from this question about Nondetection and True Seeing applies. In short, while the definition of "target" in 5e is ambiguous at best, things you attempt to perceive through spells like Detect Magic and See Invisibility most likely count as targets, even though these spells are only directly cast on yourself. And that means that Sequester's protection likely extends to divination spells that augment one's senses.

For the sake of completeness, it's worth noting that some special senses and abilities such as blindsight, a rogue's blindsense, or even just a keen sense of smell can also perceive the sequestered creature.

By the numbers

So, to summarize, I'll address your numbered questions:

1. If you believe your target is in range (and not behind cover), you can just cast Dispel Magic and target it.

2. See Invisibility won't work, but truesight would work as long as it isn't provided by a spell. (That is, a True Seeing spell wouldn't work, but a gem of seeing would do the trick.)

3. Save your flour for baking. Just cast the spell. Better yet, give the creature a good kick.

4. The creature has no thoughts to detect, since it is in "suspended animation" and "time ceases to flow for it". Hence, Detect Thoughts and other methods of locating a creature by their thoughts will not help. (Although a generous DM might rule that the "suspended" creature is still thinking whatever its last thought was before the spell was cast.)

5. Sequester already provides nearly all the benefits of Nondetection. The only additional protection provided by the latter would be against non-spell-based divination magic. (However, note that unless the DM decides otherwise, magic items aren't associated with a school of magic, so the aforementioned gem of seeing will still work just fine.)