The transmissions are Capitol Gears HY-25000. Found the problem! The alarm sensor is actually leaking, just the smallest amount. The engine room is spotless, so I know when and where oil is coming from and I'd never seen it from there, but today I see just a drop on the top, coming from the sensor. replacement will arrive tomorrow, and hopefully that does it!
"spotless" and Detroit's are incompatible. If they are not leaking then they don't have oil in them as the saying goes.
They do leak a bit, but the PO was fastidious about keeping them clean. I've kept it up. As a technically challenged person, I find keeping it clean every trip helps me know what's normal and what's not. This engine room is spotless, you could eat off anyplace in there, except the floor outboard of the engines - that's not as clean because even a mouse couldn't get in there!
Capitol gears. GOOD. I was concerned they were Allison with a bad front seal. No worries on that now.
I'm doing the oil and filter on them next week too, just to set a baseline for myself. Called the company, good guy, very knowledgeable.