Thanks for the reply. I suspect that the center cap was stolen, because one of my side markers (same side as the missing center cap, which is front passenger side) disappeared at the same time. My car was at a parking lot, so the thief was probably hiding between my car and another car next to it, and didn't have the guts to take all four.
It is impossible that it fell out while driving, because it is a screw-in type, and I found that all other center caps are still screwed in real tight. I will try to visit some local shops. Thanks again.