Everybody has their turn on the bench. Injury or illness keep even superheroes at home for a bit - heck, sometimes Bruce just declares one of the family needs a break. Could be that you've got a big test or event coming up, could be that you've had a rough experience in the field, maybe he just thought you looked tired. But every member of the Bat Family has eventually had their turn getting told that they will in fact be missing patrol tonight, and were expected to take the night off.
You'd think being left home to entertain yourself in a literal mansion wouldn't be too big an ask. It's not like there's nothing to do. But admittedly when you're used to the hustle and bustle of having the whole family in and out and about all the time, perhaps 'entertaining yourself' by your lonesome becomes more difficult. Add on that the Bat Family has a reputation for being a bit too dedicated to their work sometimes - oh, does Alfred know that better than anybody.
So he isn't terribly surprised to catch you trying to override the code on the elevator down to the Batcave, frustrated that there even is a code blocking you out. Bruce had learned pretty early-on that some measures needed to be taken to ensure that his family actually took the night off when ordered.
Alfred can't deny amusement at the thought - lord knows, Master Bruce is the worst of the bunch when it comes to such things. Pity they haven't found a way to lock him out of the Batcave reliably.
"Come now. I'm certain we can find you something to do that is not work-related, hm?"