Pulled out my fixed gear bike to ride today, and lo and behold, the rear tubular tire was flat. No sign of a sealant leak, so I couldn't determine where the puncture is. Nothing fancy; just a YJ "3 for $50" tire, but I've had good luck with those. This one is on its second or third season and still has decent tread. Felt like riding rather than dealing with it, so a swapped the tubular wheels for clincher wheels and went for a ride. When I got back, I squirted some more Stan's inside (not easy, as the valve cores aren't removable), pumped it up, swished it around, and still no obvious sealant leak. I'll keep checking, and if I find it, I'll position the wheel with the leak site down, so the sealant can pool there and plug the hole. Otherwise, if it's still holding pressure tomorrow, I'll put it back in service and hope for the best.