My Aunt is 86 slight dementia after a stroke, she has her aide move a vase after I place it, or plant boxes on the porch suddenly disappear. I see this as a power thing, and an attention getting thing, and it can be rather annoying. I live upstairs from her.
You can't set boundaries with someone who doesn't understand them.
Let it go. There are more important issues to deal with. And remember, "there but for fortune go you or I". No one chooses to develop dementia.
If it a matter of safety or significant monetary loss (my dear husband microwaved his hearing aids) then you'll have to do your best to intervene. But a vase moved, even to a very weird place ... don't sweat the small stuff.
If she doesn't like where I have put something she doesn't tell me but she asks my daughter to move it for her - now I ask and she always says yes thats fine but she still asks my daughter to move it later - I don't think you can ever understand it - it is what it is I am saddened to say.
If things are going missing you need to find out why though. If your Mum is giving things away you might want to consider getting some form of guardianships/conservatorship/POA. She wold never have wanted to be a victim of financial abuse so do keep an eye out for money and jewellery in particular going missing and as Garden Artist said especially drugs - your mum may be on medication that has a high 'street' value and if they go missing that also means she isnt taking the right meds.
I hope that helps you hun xx
Plant boxes disappearing is another issue - these are heavy so I doubt your aunt could move them. What does the aide say about this?
If there are serious disappearances, such as of financial items, medications, etc. I'd be really concerned.
But elderly people do have a tendency to misplace items and have no idea what's happened to them.
Not to be critical, but how does the movement of items annoy you? Does your aunt become upset that she can't find things?