I was on the bus...

[sally]

As far as I can tell they weren't even aware of it.

It's still sexual harassment. Intentional behaviour that is threatening or disturbing, whether they are aware of it or not. Just think of when someone is being stalked they are not always aware that the predator is intruding on them.

Oh my fucking god. It was just some old guy that got up to take a gander at some chicks ass.

Calling him out on it isn't going to make him any less of a pervert, it's not going to prevent a rape and in the end the only damn thing it did was make CN proud of himself for doing fuck all other than to draw attention to the woman who most likely didn't want it.