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.