Yep, turned out to be missing the hole in the thermostat. I got a new one that isn't a custom brown color. Thanks for all the input.
So why the custom brown? Rust in the system at some point in its history? You ran (only?) water in it for a time, didn't you?
I've never seen a thermostat discolored like that in a good system, but maybe I've lived a sheltered life.
FYI, that's where my temp gauge runs too, coincidentally.