Andre Majorel schrieb:

....
What I don't get is how, after shorting the plugs with a 2-cm
long piece of wire, switching to "Velec" mode (270 k input
impedance), switching off all monitors and fluorescent lamps in
the vicinity and going outside, you *still* get 9 mV.

I have an old (~1980) 4.5 digit true rms DMM; given accuracy for
ACV: 0.25% rd. + 20D!

300 mV AC range, input shorted - display 000.12 (mV)
300 mV DC range, input shorted - display 000.00 (mV)

Your reading could be normal for this type of DMM. What says the
manual?

Just my 2 cents

Reinhard