For me, this is only an 'opportunity' if the guy is saying "ticket please" and not just walking.
I'd disagree, a ticket machine in working order is an 'opportunity to pay' but it doesn't ask you for a ticket when you walk past it.
I think someone in uniform with a ticket machine walking through a train is indeed an opportunity to pay, regardless of whether they are challenging a passenger directly or offering to check tickets from recent boarders.
My 1 car 158 train home yesterday stopped somewhere fairly rural and the guard came through, a little frustrated as he'd seen ONE passenger join and no-one was 'fessing up' so I politely pointed out the lady opposite me, explaining as an ex inspector I wasn't going to ignore it.
When travelling to work many years ago as an inspector manager I regularly got up from my seat with a sigh (after the guard had 'called over' unsuccessfully) slapped my badge on and did a full check in the carriage.
I have some sympathy to those that feel their ticket is checked 'too much' but unfortunately for them -regular fare evaders hide in the camouflage that reluctant ticket showers provide and bump up everyone else's fares and are, IME, often the sort that can very easily afford to pay but choose not to, ironically because they see others not showing tickets when asked!