I was on a group ride on Saturday from Pizza Port San Clemente to Pizza Port Ocean beach (stopping at Pizza Ports Carlsbad and Solana Beach on the way...). After I got out of Solana Beach, my front tire was flat (Marathon Supreme). Did a quick visual check for sharps, didn't see anything. Went to pump it up, saw the valve was open, so I think somebody let my air out. However, I had been noticing that my front tire was beginning to feel a little soft. If I left the valve open, is it possible for road vibration bounce the valve to burp a little?
In any case, I pumped it up, and it held for the remaining 20+ miles. (However, I did pick up a nasty staple that took the rear down...)