Joey wrote

Suppose someone visited your office or home and
tried to make a voice recording using a hidden recorder.

If they used a older-style dictation machine based on tape
then you could detect the electromagnetic transmissions
from the dictation machine when it was recording.

In practice that was always a lot easier to say than to do.

But how would you detect if someone was secretly recording
with an MP3 player that recorded to flash memory?

In theory any electronic device will produce some
EM so at least in theory that can be detected.

In practice there is so much of that stuff in any
normal room anyway that its just not feasible.

Is there some transmission which could be detected?
Perhaps some low power ultra high frequency from chip refresh cycles?

Fraid not.