Many thanks for all of your answers and valuable tips so far!
SMPSU based wall warts usually have an EMI suppression cap between primary and secondary which passes a low current.
Well, this reminds me of (very) old tube amps. In addition to having two pronged mains chords (Safety earth simply wasn't provided in these times) there was a capacitor wired between one of the mains leads and chassis ground to provide "noise suppression". Nowadays this "feature" is known as the "death cap", as this would be very dangerous indeed if it failed short.
Well I HOPE the EMI suppression cap in a SMPSU lets pass a few "user safe" volts at most if it failed short.
If it should be of a certain type for safety reasons, I would count on our friends from the "Won Hung Lo" factory to cut some corners.
It seems, to be really insured against unexpected earth references, I should only use battery operated equipment!
I didn't really understand your measurement tips concerning "Detection of mains/earth reference in mains operated appliances".
As I have expected, there are some problems .... ghost voltages, references via capacitor ....
Could you give me a more noob friendly description and tips how to measure?