yeah, u're right. My friend is like this. And his officemates are somewhat forced to stay late also coz of his habbit. They used to have a straight shift (until 5:30) but now, since the Boss notice that it's ok for them to work on extended hours, they go home 9 pm or a little before that.
Working on extended hours only means one thing: there's a lot to do OR u don't know how to manage your time.
I always say "i still have a life after office, got to go now"