Well the shop had my truck all day, and couldn't find a definitive leak. They cleaned the underside of the engine and crossmember, ran it, drove it, and put it back on the lift. Nothing...just a really clean engine. I poked around with a flashlight while chatting up the mechanic, and we both agreed that there was no obvious oil leak.
The thermostat was weeping coolant ever so slightly, but that's it.
Shops advice? Drive it, check it weekly, and it may or may not reappear. Walked out for $20 to clean it and have the coolant system pressure tested. Pretty fair, they could have replaced the seal for a lot more, and I wouldn't have known the difference.
Onto other projects for now....