Given the average number of people on most VT services, can guards really remember what tickets people have shown them?
When travelling on a combination of tickets, I always only show the tickets that make my current presence on the train valid. But guards don't seem to remember that I was "supposed" to get off at a certain station, and only ask for new passengers to show tickets. I have observed people taking advantage of this quite blatantly, by "buying short" and then staying on the train, and they don't get challenged by the guard later on.
I have also noticed that on busy trains, especially longer ones, if guards are not in "check every passenger" mode they only check passengers who proactively present tickets. I also seem to be invisible to some guards.
Normally my tickets are in my wallet in my trouser pocket. It's quite a faff to get them out. If I expect to be checked, the tickets are on the table. But sometimes I'm tired and can't be bothered, and unless I've boarded at the first station, the guard often just walks past saying "tickets from ___ please" but ignoring me even though they should have identified me as a new passenger.
The following has happened twice now: I'm sitting by myself at a window seat on a table of 4, then a group of 3 gets on and surrounds me. The guard is handed 3 tickets by one person, they mark them, I am holding my ticket and about to stick my hand out, but the guard just moves on. I and the guy opposite me shrug our shoulders at each other.