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