this just started happening to a friend of mine on tour. she got multiple flats in different spots on the outer part of the tube. we figured it out on her last flat. there were tiny bits of glass embedded in the tire you could not feel or see until we spread the rubber around some areas with really small nicks in the tire. there were two or three areas we picked really little bits of glass out of with tweezers that probably kept digging into the tube that we couldn't detect by running our fingers around the tire but would probably stick the tube after being brought up to pressure and ridden for awhile. drove her nuts until she figured it out...