I'd read about it on this forum, but today it happened to me -- a co-worker let the air out of my rear tire.
This was the first day I commuted with my new Conti Grand Prix 4 Season tires as well. At first I thought I must have punctured it somehow on the way in, shortly before locking up at the bike rack. However, it wasn't totally flat, and hey, these are supposed to be tough tires right? The guy I work alongside gave me a ride home, and first thing I pumped it back up. OK, holding air it seemed. Grabbed my frame pump just in case and hit the grocery store. Got back home and the pressure still felt solid. Let out the air, removed tube, and did the water test. No bubbles. Co-worker theory is looking a lot more plausible now.
I work at a relatively small company and I know just about every employee. There's one guy who has a grudge on me, because I got too close to the lady he was trying to woo about a year ago. I'm not sure what to do now.