Welcome to Psych Central. Sorry your things are disappearing.

Sounds like an inside job. What about moving to a new apartment? Who has the key? Did you change all the locks including cellar or shared doors with others apartment. What about a window with no lock?

It is sad to losing things, but it is an opportunity to let go of the things you cling to. Talk to your therapist and see if they can help you process this odd experience.