Nothing'* ever easy. My coolant keeps disappearing. (Good thing I popped the hood when I got to the restaurant!)

No leaks under the car when it'* off or idling, nothing looks wet, and my oil still looks like oil. I'll check for condensation under the oil cap when I leave in case my UIM suddenly cracked.

It'* got to be something I touched, but nothing looks wet.

Never mind. I drove back home and saw a spot where the snow wasn't sticking. A taste test says that coolant is dripping from the petcock. It must be dripping slowly enough that I don't notice it until the reservoir is halfway down.

Just to verify, because I normally drain via the radiator hose: do you tighten the petcock valve to close it (like a bolt or a screw), or is it one of those that you can turn in one direction to both open and close (like you sometimes see on some glass pitchers)?

EDIT: I don't recommend tasting coolant. It is highly toxic. I only did it because it was too dark to see green and too cold to smell.

It'* tightened like a screw

Summary for posterity:

Replaced the idler pulley, and that worked for a couple hours.

Replaced the entire belt tensioner assembly (including coolant elbow), and that worked for a few minutes until the stronger arm stretched out the belt. (Yes, this means I now have an extra idler pulley and bolt.)

Finally, I replaced the belt (like you're supposed to when you replace the tensioner) and all is well.