Or maybe not a leak, per se. My first floor toilet began running constantly
between flushes 3 days ago. Not full blast, just a trickle to remind me of
how I'll be spending part of my Saturday. Or so I thought. Just noticed a
small puddle in the cellar below the toilet. It's the sewage pipe that's
wet, but the puddle smells clean. My first thought was that the supply pipe
got so cold from running constantly that what I was seeing was condensation,
but as far as I can see (because part of the supply pipe's hidden), that's
not the case.

Next thought: The cold water contracted part of the sewage pipe assembly
enough to leak. But again, based on the lack of smell, that's not it.

Nightmare thought: Wax ring. I don't want to think about that.

There's little or no condensation on the tank itself, and the floor in the
bathroom is dry. Luckily, it's not humid here today.