8 flats in two days was driving me nuts. Nothing I could see in the tire, nothing on the wheel, rim tape was intact. Yet every tube lasted an hour if it was lucky. Then today I sat down in the direct sunlight and checked every mm of the stupid thing. Finally found it. A black bit of metal just large enough to sit flush in the tire when it wasn't under pressure. Thus almost invisible. When the tire came up to pressure it of course had enough of a sharp point to deflate tires slowly... really annoying.
All that and I'm gearing up to ride 260km in one day for the first time on Saturday. I was about to just throw in the towel and get a new set of tires and I finally found it. Whew. Hopefully there is only one.