Anyone ever had a mystery flat before? Did a 15 mile ride w/ my son on our MTBs on MUP 2 Sundays ago, no problem. 4 days later, walk past my bike in garage to find rear flat. Check the tire all around w/ a flashlight, nothing found. Remove the tube, pump it up greatly, no leak detected, no loss of air after 30 min. Finger-sweep the inside of the tire, nothing found. Check the rim and rim tape, no problem found. Put same tube back into same tire and pump up to 80 PSI. 4 days later, absolutely no deflation at all. No loss of air to date.. What gives!!!!