I have a similar story.
I had a friend who lived a few houses away from me. We'd always hang out and be at each other houses a lot. Sometimes my "friend" would "accidentally" walk away with a guitar pick, little metal Risk figurine, etc. Nothing of value, but I still got pissed that he would take stuff from my house. Well eventually over the years I noticed that I couldn't find my Palm m100 nor could my dad find his skinning knife that he kept in the garage.
Fast forward to a couple years after the Palm m100 was missing, I walk over to my friends house, and his dad answers the door. He tells me to go to the back because my "friend" is in the hot tub. And that's when I noticed him playing around with my Palm m100. To make a long story short, he denied it at first and later his Dad bought me a newer, nicer Sony Clie.
I still think he stole my Dad's fish skinning knife as well.