I had the weirdest sidewall puncture on my BMC last weekend. I had the rear wheel off and in my lap while running a rag through each of the sprockets when out of nowhere, I hear a small hissing come from the tire. While I was holding it. Move it around until I find there's a teeny tiny puncture in the sidewall with some scuffing around it, must have ridden a bit too close to something. Thing is, last time I rode it was at least two months ago and it held air the whole time since. Moved the wheel so sealant could get into the puncture and I got lucky, the stuff didn't spray out and I had just aired up the tires before the leak made itself known. Tiny bit of sealant poked through and immediately dried, no mess.