Well, I took one of the suggestions from this thread and I saved myself the trouble of taking the wheel off the tractor. I just grabbed a big Rubbermaid tote that I have laying around and raised up the tractor's front end with the loader, then put the tote underneath the wheel and lowered the tractor's wheel down into it. I filled the tote with water until it covered the bead and then turned the wheel by hand until I found the leak. Sure enough, it was tiny and invisible, but I found it and patched it. Ultimately, given the location of the hole, I could have just poured water gently into the tread and it would have held enough water to find the leak. This is something that folks might consider as a first step, before they start pulling tires off the tractor or going to greater lengths. Yeah, I know the "paint soapy water onto it" trick is supposed to work, but I have often had it fail to find a leak that submersion later found. Maybe I'm doing it wrong.