Not very often.
When I came back from a business trip two or three years ago, my mom picked me up at the Metro station. We decided I would "sneak" in the house.
Ozzie was in the kitchen at the time (had to do some potty-training reinforcement) so I was able to slip in and my mom just greeted him and let him out. I stood in the hallway quietly and he almost had a heart attack when he saw me.
It's the only time I've ever been able to do that.
He always knows when I'm home.