I've seen many trains that have poorly marked first class areas. Some of the ex-LM 321s that FCC got (and by the way, I know these aren't on the TL side but plenty of people are pointing out that there are some 377s that are much the same) had just the old-style BR '1' numbers on the grey background. And that's it. I guess that's how LM delivered them after removing all of its own material.
Obviously FCC would/will eventually put on their own stickers, which are much more obvious, but perhaps they were just going through the process of having the anti-etch film on the glass, and so on.. whatever the case, two very small '1's and antimacassars that will be invisible if people are in the seats does not, to me, mean it's obvious to all.
I'm the first to say that those who knowingly go into first - even to stand, or believing that perching on an arm rest isn't really being in there - should be done, but I'm not going to pretend that some people aren't genuinely caught out.
In fact, I'd say there are a good few people that I've seen come into first class and take a few minutes to realise (even when marked more clearly) and then quickly left. In these cases, had an RPI jumped on them the second they entered and sat down, once the doors had closed, they'd be in big trouble.