"tim \(moved to sweden\)" wrote:

The discussion had moved on to a question about the legality of
them demanding a ticket check from someone that they had seen
come out of an 'exit only' route (you seem to have snipped this bit
and returned only to the original question). As they would not
know if the person had an Oyster at this point, how could this be
a reasonable thing to do?

They could turn up the sensitivity of a reader, and, if directional
enough (or the target was sufficiently isolated), (try to) read
the oyster from a distance.

#Paul