Spooky multimeter AC reading using 1 lead

Question

I'm fixing an ultrasonic cleaner board that isn't working. There's a small transformer that drives the transducer. When I set my DMM to AC and touch both leads on its output, there's no reading. If, however, I touch only one lead, leaving the other one resting on the bench, I get 3.5V! Spooky!

What could it possibly be referencing? Could the high frequency 40KHz going through the tiny transformer induce a voltage on the other lead sort of like an antenna? It doesn't seem plausible. It's freaky stuff like this that makes me feel so small and inept in the face of electricity. It is incredibly counterintuitive.

ASIDE: Some additional spookiness. There's a small unmarked disc capacitor across the transformer output terminals wires. If I probe with both leads where the terminals are soldered together with capacitor, I get a reading of 3.5V.

Answer 1

High frequencies, capacitive coupling, and high impedance multimeters will easily end up in weird readings.

There may very well be some AC signal which the multimeter can measure and show a result of some value, even if that value is not entirely true.

Answer 2

It is entirely plausible for your test leads to pick up currents from the circuit through air like antennas, especially with HF power electronics such as your cleaner. Really high power high frequency devices (for example a plasma cutter) have enough radiative power to do things like literally start typing garbage on a computer nearby by inducing currents inside the keyboard.