I had a stamp stuck on an envelope that was not Franked and so thought
had never been used. So i cut around it and stuck it to a new envelope
and posted it. but i got a note from the post office that i needed to go
in and pay extra.
As an experiment i used a stamp that i knew for sure had never been used
but had been stuck on an envelope. So i again cut around it and posted
it to myself and it arrived.

Does the post office have some way of detecting if a stamp had been used
through royal mail even though there is no visible sign of Frank Marks?