The case is the following. Over 6 months ago, a coworker required a USB stick for work related stuff but we hadn't one ready for use. By coincidence, I had a private one with me which had no data on it, and I intended to keep it. So I lent it to him and said it doesn't matter if he needs it for 1, 2 or 3 days. On multiple occasions, I asked for the stick and he pretended to have forgotten about it and it is at his home. As he has a Ph.D. and I was interested in his thesis which he wanted to give me, I told him (when he was pretending one more time he forgot about the stick), that there is no hurry, he can just put the PDF's onto the stick and give it me back then. (Which, of course, just was a pretense to recover my stick.) He evaded this for a few days just sending me his thesis by mail. After that, I forgot about it. But yesterday I was searching for some work related documents and didn't realize I was opening his private drawer of a cupboard. And there was my stick. I left my stick in there as picking it up would expose that someone was in his private stuff.
But the main point is:
How to react here? He obviously lied to me and kept my property into his possession. I know that when he is at work lays down some stuff (which is distracting for his work), into his drawer so he faces every working day my property well knowing that I asked multiple times for it (After clarifying via comments that this is important info: note the USB stick has a keysign attached to it, with a label that holds my name handwritten on it). I don't know why he could do this, as a 8 GB stick isn't that much of a value. And when I was thinking he really just forgot about it, I was assuming good faith and therefore had no problem by just letting it go. But now when I know he obviously lied to me, with the aim to keep my property in his possession, I feel like I should do something. But what? (escalating it to superiors is nothing I'm aiming for, while this is a pretty bad attitude on his part, it's still a good climate in the whole team.)
note: I wasn't able to talk to him since this happened. So probably when I ask him the next time he will just remember that he brought it back to work again a few weeks ago, and just hadn't the opportunity to hand it over again. But for this post let's just assume I asked him after this and he just said he has it at home and will bring it me in the next few days. (What as I now know would be in that case a lie)