Thanks for the info.
I'm still waiting for a repeat of the symptoms to do an actual voltage check.

When the compressor/fan wasn't running the thermostat was showing cooling on.
There's no compressor operation and no fan operation. Even after power cycling at the fuse box and thermostat (delay). I can look down on the unit condenser fan through a window within 5 seconds of the thermostat kicking on cooling.

So most likely it's a contactor, since the thermostat has power without the batteries during the event, and is intermittent (cycles fine for hours or days before failure) right?