I have a Conext (by APC) model CNB-325 battery backup power supply (not a real UPS) that shows funny voltages when the power cord is disconnected.
Backup supply plugged into AC outlet;
hot-neutral: 120 VAC hot-ground: 120 VAC neutral-ground: 0.7 VAC
Backup supply unplugged from AC outlet, computer used as load:
hot-neutral: 115 VAC hot-ground: 58 VAC neutral-ground: 58 VAC
Backup supply unplugged form AC outlet, no load:
hot-neutral: 115 VAC hot-ground: 19 VAC neutral-ground: 60 VAC
A different model Conext, model CNB-300, behaved similarly, only the no-load voltage on battery power was 90 VAC neutral-ground and 40 VAC hot-ground.
I didn't get such wierd voltages on battery operation with a much older APC BK-300 (much larger battery, all-metal case); hot-neutral and hot-ground both measured 118 VAC, neutral-ground was slightly under 1 VAC.
I measured with both a Fluke 73 digital meter (true RMP) and an analog meter with 10K/volt sensitivity. The analog meter gave very different voltages with each backup supply running off battery, but I assume it was caused by the AC not being a real sine wave.