You said that the clients are away right now. So I assume this house is probably at a very low energy use. If that is the case, it’s still possible that the neutral is being lost and the cable is able to handle the small amount of current. Even then, if leg A is using 10amps and leg B is using 8amps. That leaves 2amps going down the cable line if the neutral truly is lost.

Also you could put a camera facing the UPS with a clock, if need be, to record when it goes to battery and for how long. Just curious, what kind of UPS doesn’t log to a computer that has a display?

KOT