Joey wrote
kony wrote
Joey wrote

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

Ok, why do you care?
Don't have sensitive conversations with anyone you can't trust,
if you're going to be saying anything that shouldn't be recorded.

I say things in my conversation which i do not
want to be broadcast in the public media.

Then you have a problem for which there
is no solution given the other detail below.

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.

Doubtful, and not worth the bother when you
could just scan them or search them physically.

You say, "search them physically"? I have to tell you
that I don't work in the sort of environment where
that kind of behaviour is considered acceptable!

Then you have a problem.

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

Scan them or search them.
What makes you think you should detect it?
Don't they have a right to a recording of
any conversation they're participating in?

They have the right to record themselves. I have the right
not to have my words broadcast. Unfortunately obtaining
redress can be expensive. So prevention is beter than cure.

Sure, but it just isnt practical.

Is there some transmission which could be detected?

Do you mean "wild hypothetical way that some future technology
or extremely expensive equipment and controlled environment
could detect", or do you mean, practically speaking?

The latter. I am talking practicalitites ere.

The short story is that it isnt practical to do what you want to do.

Perhaps some low power ultra high frequency from chip refresh cycles?

... and you're discriminate this from everything else,
all other HF noise, how? Get a baseline maybe,
but noise is random unless a controlled environment.

If necessary I could have my PC turned off and my phone put further away.

Wouldnt be enough given the minimal leakage from a
decently designed solid state MP3 recorder. In spades
with one that has been designed to be undetectable.

But is there any leakage from solid MP3 recorders?

Corse there is, but the level will be so low that you wont be able
to find it with all the other stuff around that is radiating, even if
you turn off everything in your house when a visitor shows up.

You just need to have everyone strip down nude and then do
body cavity searches, X-Rays, exploratory surgery, then never
let them leave so it doesn't really matter if anything was
recorded so long as the environment has sufficient RF shielding.