@tjcook,
Wow, that’s a big single circuit source for an Always On. Just to confirm - when you say you are using the circuit-by-circuit approach, you are flicking off that breaker when the house is relatively quiet, and the Power Meter drops by 750W, right ? I ask, because the Always On bubble won’t respond to a change like that for a substantial period of time. If so, two thoughts:
It’s not the Sense monitor borrowing power - the monitor only uses 2-3 Watts.
The giveaway should be what doesn’t work when you turn that breaker off ? But it might be trickier than you think - it took a while for a couple of community folks to find mystery heaters, that they didn’t know existed.
One additional way to get ideas - try the community device library and set the tags for the approximate wattage of the mystery device. You might get a hit: