all sensors for grey and black were empty and all battery, lpg and FW were full when i left the dealer.
since it had been only 12 hours after we hooked up there were no showers, dish washing, etc., to account for even a small amount of grey tank use. i also checked all of the faucets, sinks, showers and even the outside shower (even though i know this doesn't drain into the grey tank) but didn't notice anything wet, dripping, etc.
i know it's the weirdest thing. i was thinking perhaps a valve just wasn't seated quite right in the overall water system loop since it appears ok now but i wouldn't even know which 'valve' connection could even trigger this kind of issue. other than that it could have just been a 1x sensor issue from the beginning and once i drained it everything is now working properly.
the good news is i caught it in time. the bad news is i will need to be more diligent and check all sensors more frequently.