Can Magic Mouth enchant carried objects to prevent ambushes, pickpockets and more?

Question

Magic Mouth is described as being cast on something that isn’t worn or carried - but seems non-specific what happens after:

You implant a message within an object in range, a message that is uttered when a trigger condition is met. Choose an object that you can see and that isn’t being worn or carried by another creature. Then speak the message, which must be 25 words or less, though it can be delivered over as long as 10 minutes. Finally, determine the circumstance that will trigger the spell to deliver your message.

When that circumstance occurs, a magical mouth appears on the object and recites the message in your voice and at the same volume you spoke. If the object you chose has a mouth or something that looks like a mouth (for example, the mouth of a statue), the magical mouth appears there so that the words appear to come from the object’s mouth. When you cast this spell, you can have the spell end after it delivers its message, or it can remain and repeat its message whenever the trigger occurs.

The triggering circumstance can be as general or as detailed as you like, though it must be based on visual or audible conditions that occur within 30 feet of the object. For example, you could instruct the mouth to speak when any creature moves within 30 feet of the object or when a silver bell rings within 30 feet of it.

If Magic Mouth is cast on an object that isn’t carried - but then that object is picked up and carried - does Magic Mouth continue as a spell?

For example, could you have a necklace of beads each with a different Magic Mouth watching for some condition within 30 feet of you? “Warning! Someone is trying to pick pocket you.” Could you place the beads in a circle around your camp to be picked up, carried and reused. “Pssst...something big...look over here”

Answer 1

Your question sort of answers itself when you look a this part of the spell

worn or carried by another creature

The language of the first paragraph denotes what you can cast it on at the time of casting, once the ritual is finished it doesn't care as long as it's not dispelled or destroyed. It's just not designed to be cast on other people's items while their in their possession. This does allow you to say, take an item while someone is sleeping, enchant it, then put it back.

The only other spell I could find that should affect attended objects expressly does not, so it seems like the designers don't want you to affect attended items.

From Shatter

A nonmagical object that isn’t being worn or carried also takes the damage if it’s in the spell's area.

Answer 2

The answer is yes.

The spell states the object magic mouth is cast on cannot be "worn or carried by another creature" PHB pg. 257 (italic emphasis is mine). There is not a prohibition on casting it on something you, the caster, are carrying.

Since you can cast it on your own possessions you are carrying, it follows the prohibition really isn't about not being able to cast it on something that can be carried. It's about not casting in on something that someone else is carrying and hence surprising them now or later.

Keep in mind it must a "visual or audible condition" so attempting to pick your pocket doesn't qualify as it requires knowledge of intent. Maybe it is another party member retrieving something from your backpack with permission or maybe walking on a crowed street and someone almost bumping into you from behind. Someone removing something from you might work however.

Answer 3

As stated in other answers, yes, it can be cast on things carried by you or that were unattended but are now being worn or carried by other creatures. And as @Bob stated, it can't determine intent, so you need to look at unequivocal conditions.

So... pickpocketing is probably the easiest to deal with if you can identify the most likely item to be pickpocketed. Let's say it's your money pouch on your belt, the sort of thing that there would be no legitimate reason for another party member to take away from you. You need to be able to take it off yourself to bathe etc, so cast a Magic Mouth on the pouch and set the condition that if they are taken more than 5' away from you by another creature or force (don't forget telekinesis-type effects) then they can loudly say "Hey! A powerful wizard's money pouch is not allowed to go on journeys without permission. (Pause 10 sec) I AM OVER HERE! (Pause 10 sec) I AM OVER HERE!"

Ambush while on the move is a lot harder to deal with because of the difficulty of setting unequivocal conditions. To use your example from the question, if your party are all medium sized creatures and you set a condition of:

if there is a large or bigger creature within 30' then say “Pssst...something big...look over here"

then it will trigger every time you approach a horse. Even if you can specify particular type/s of enemy (eg goblins or giants or...) then it may compromise your surprise if you are trying to sneak past them or would have surprised them in an encounter battle. Not to mention the irritation factor in a prolonged battle, searching bodies, interrogation session - every few seconds your warning device will be saying

"Did you know there are enemies nearby?"

"Did you know there are enemies nearby?"

"Did you know there are enemies nearby?"

Your idea about beads around a camp sounds good, just set the first part of the condition as "when this object is not wrapped in a cloth" so you can choose when to take it out and "arm" it. However, the inability to determine "intent" means it is not the equivalent of the Shadowrun spell Detect Enemies while mobile.