After some more experimentation, it seems like these components are very sensitive to input voltage. It's alleged that they want anything from 4.5v to 20v, but the tirggering behavior is pretty erratic for every power supply I've tried from 5.5v up to 12v.
They seem to work best (most stable) when powered by the 3.3v pin from the NodeMCU board. Hmm. In my note yesterday, I forgot to mention that. Actually, I didn't even notice until I was looking at my breadboard this morning. It's one of the zillion things I tried.
I guess if I end up using the HC-SR501 units in the project, I'll have to select the specific ones by testing them with the voltages I have available (which will be 3.3v, 5v, and 12v). Maybe the AM312 units will be more consistent.
Another thing.... Sometimes, after getting my 2-3 seconds of HIGH for motion detection, the signal will go low for 5-7 seconds and then mysteriously go high again for another 2-3 seconds. That won't make too much difference for this garage light project since the lights will still be on from the initial trigger.