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.
2007 Pontiac G5
1999 Pontiac Grand Prix GT - Stock engine ate transmission.
1997 Pontiac Bonneville SE - Cammed engine ate transmission.