Maybe knowing where your stuff is gives you a sense of things being safe and orderly. It sounds like this is related to some kind of anxiety problem. I've read that obsessiveness is related to anxiety . . . an attempt to control it.

Often, lost objects will turn up when you are doing a thorough job of cleaning. I find that works better than trying to hunt for something.

That's too bad that you think your mother might have hidden your MP3 player. Why do you think she might have done that? Sounds like your relationship is not too good. Maybe you could talk with her. Have you ever told her that losing things makes you feel very disturbed?

I hope you find it soon.