On international-international flights, a couple of times now I have walked straight out of the airport for a potentially expensive cigarette and a wander around for an hour. Despite not having TUUA included in my onward tix (as the plan was I stay airside) I have scanned my 2D barcode and got the green light to get back in without paying.

I am fairly convinced that the scanners only check that it is a 2D barcode and not that you have paid TUUA