That’s my impression, but if all you have to do is measure the current draw, then you don’t need the third wire. So I’ve built a basic ADC circuit to do that, but it’s pretty crude.

In the original monitor, it was quite precise, and so I traced the circuitry as above off that other wire. I don’t know enough about how all that works to replicate it and I can’t seem to find much about similar sensors to learn more.