I had that "problem" in my old truck once too, but it turned out to be my co-workers messing with me. They took a set of my keys once, and they would move the seat, or turn up the stereo, or move it in the airport parking lot. The joke finally culminated with them putting it for sale for way too cheap on my front lawn while my wife and I were on vacation. She was none-too-happy!
Check your friends...
