I had something similar happen to my hybrid last season... tire kept going flat (total of 7 times!) after patching or replacing the tube. Everything seemed to be OK, nothing stuck in the tire - I checked 7 times.
Turns out that the rim tape was worn out (stretched?) and would "creep" when the tube was being re-inflated, exposing the tube to the spoke holes in the rim below. Replaced the rim tape and everything was fine after that.