I, and many others spend their time kicking folks out of first class, but sometimes its not possible to check every train, all the time.
eg. We work a 12 car that runs from Vic to horsham, where it splits. In the rush hour its not always possible to get through it all, plus you have to be in the middle at Crawley for sighting, and then front 4 for Littlehaven. So the last time I worked it, only manged to check first class in 4,5,8,9 and 12. So if you were in coach 1 for eg, and though there was not someone who was first class (and you can NEVER tell just by looking), you would be hrumphing and thinking, typical, staff never checks first class.
Just an example.
And then there are many DOO services.
And in a aside to your, been there all the time, I had a pass on my train who was PF'd for sitting in first class, 30 secs after we had left Victoria.