GFCI Outlet Neutral Shows Hot
Without the neutral attached to the receptacle the terminal is floating. So, taking a voltage reading (comparing it to ground) you could get different readings and 20 VAC is not surprising. The current you were measuring was the leakage current through your meter.

It wasn't actually drawing any power during your test since you hadn't completed the circuit and were just taking a static voltage reading. The device may draw micro amps of current to operate the circuitry while it is operating normally. Modern test equipment is very sensitive by design. This is why a static voltage test on a car battery doesn't really show the condition of the battery. A better test would be performed with the headlights and defroster on.

The main goal of a piece of test equipment is to measure the circuit without affecting the circuit.