Thank you everyone for your comments and observations. Whilst it wasn't me who was naive enough to believe that property could be stolen in a security check area, it probably could have been me at any point, because like my friends, I trust people (and I trust the system). I now know better, and so do they.
These people had never travelled by air very much. They were mesmerised by something happening to them which they were not familiar, in a strange and confusing environment with a foreign language, different mannerisms and (to be factually frank) a perceived authoritarian attitude (it was in Germany).
I've found myself in a similar situation only once at NZAA where I was really rattled by a confrontational woman who demanded I empty my entire camera bag - and right bloody now!. It was unexpected; I was confused and offended by the attitude, and in that state I could have easily missed anything else which might have been (and was) in another tray. It would have been an ideal opportunity for a quick grab by an opportunist.
And on the other side of the ledger, I've actually seen a security person racing some 100 metres after a forgetful pax with their camera which they had apparently forgotten to collect in YSCB.
I've gained a few tips from this discussion and I thank you for that. If I may, I share one which I employ. On all of my camera cards, memory sticks and computer(s) I have written a short text (txt) file with the filename "Owner_Details_if_found". In it I have listed details where I can be contacted if the device comes into the possession of someone else who cares - i.e. I lose it. If they really want to return it to the rightful owner, they have the information to be able to do it.
My friends are wiser now. But they will continue to hurt for a very long time because quite simply the system let them down - it failed them in a reasonably expected duty of care to ensure that the property of the travelling public is safe whilst undergoing inspection in a security area for which they (the security staff) are responsible