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In 5th edition, would detect magic reveal illusions such as invisibility, disguise self, etc. by the rules as written? While it would not reveal the nature of the illusion, it would reveal that one is present I believe.

I know some illusion spells refer to true seeing and see invisibility which are clear that they bypass illusions.

I just imagine this can be problematic as the warlock can cast detect magic at-will at third level.

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By strict RAW, you can sense the presence of illusions of all kinds, but you can't actually see exactly where they are, or that they are illusions rather than just some kind of magic in your vicinity. From the description of Detect Magic:

For the duration, you sense the presence of magic within 30 feet of you.

Simple enough; if there's magic, you can sense it.

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, and you learn its school of magic, if any.

If a creature or object is covered by an illusion, they're not a visible creature or object, and the illusion itself is neither a creature nor an object, so you can't see its aura. An illusion that is not being "borne" by a creature or object is likewise not a visible object or creature, so you can't see its aura, either.

Where things get messy is if there is a creature or object that "bears" an illusion but is still visible. However, I can't find any cases that would fall under this category. For example, you could use Minor Illusion to cover someone's head, but that is a freestanding illusion that happens to be around a creature, rather than actually being borne by that creature.

As far as whether you want Detect Magic to function by strict RAW in your game is concerned, given the way you've phrased this question I assume you're more worried about it being too powerful than the opposite, so this should suit you.

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    \$\begingroup\$ How is an illusion an object? It even says "you create the image of an object". The image of an object is pretty obviously not an object itself. Also, you seem to treat illusions and invisibility inconsistently. \$\endgroup\$ – Miniman Mar 17 '15 at 12:21
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    \$\begingroup\$ I would imagine that any magical disguise is probably in the Illusion School, and that any creature so affected would suit the description in Detect Magic. That is, a magically disguised but still visible creature will probably have the aura of Illusion Magic. That, and a similar effect on objects, are the only things I think one would be able to notice the aura of. \$\endgroup\$ – Javelin Mar 18 '15 at 0:07
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    \$\begingroup\$ I like this answer. @Miniman One could argue that disguise self does not make one visible in a kind of physical sense (like the light bouncing off you is not observed) but i think the creature that is disguised is visible (and thus can be attack and subject to normal consquences of being seen) and thus you can sense an aura. This answer resolves my issue about invisibility \$\endgroup\$ – Drob Mar 18 '15 at 8:44
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    \$\begingroup\$ I think you're turning too heavily on the phrasing of the spell. Is tangibility a requirement? Is magical fire detectable? How about a wall of force? How about a leomund's tiny hut, or is that only detectable to barred creatures? It feels to me like we're altering too much of what I expect the average player is going to perceive as obvious functionality: detecting magic should detect spell effects. I would rather a spell effect be considered an object -- an undefined term as it is -- than have obscure functionality for detect magic. After all, Nystul's magic aura exists for a reason. \$\endgroup\$ – Bacon Bits Mar 22 '15 at 15:02
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    \$\begingroup\$ @BaconBits In a question tagged RAW, the phrasing is all that matters. What you want the spell to do or think it ought to do is entirely irrelevant. \$\endgroup\$ – Miniman Mar 22 '15 at 20:07
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Yes. The detect magic spell allows you to "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, and you learn its school of magic, if any." (Source)

There is nothing to indicate that a spell of the illusion school would cover up this aura. Even with an invisibility spell, you become aware that there is an illusion aura in a certain space. You don't learn anything about the object on which the aura is placed (except general size perhaps) and what kind of spell it is, but having an illusion aura hover in mid space is a very strong hint that it most probably is an invisibility spell.

A person under the effect of an invisibility spell could still attempt to hide normally to avoid being within the sight of the spellcaster, but he would have to be behind cover that can block the aura. (1 foot of stone, 1 inch of common metal, a thin sheet of lead, or 3 feet of wood or dirt.)

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    \$\begingroup\$ While you can detect the presence of an Invisibility spell, you cannot see auras of invisible objects. Detect Magic does not necessarily reveal the size of an aura or the nature of the spell, and only reveals the school of visible auras if the caster uses an action to perceive auras. Blocking the aura with cover would prevent the caster from detecting invisibility altogether. \$\endgroup\$ – David Reeve Mar 17 '15 at 11:00
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    \$\begingroup\$ How did you reach "you can see an illusion aura around an invisible creature" from text that said "you see a faint aura around any visible creature or object that bears magic". Invisible creatures are not visible, so why would they generate the aura? \$\endgroup\$ – Yakk Mar 17 '15 at 18:44

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