It happened to be one of those days where the Joker was incredibly bored. To try and cure this he takes a wander into one of the nearest apartment blocks and knocks on one of the doors on a higher level. He just wanted to see the person on the other side get freaked out by the fact the clown prince of crime is on their doorstep.
He would either take the hostage just for fun or just simply walk away. If they called the cops, so what… He could handle himself by now.
He walks up to the door that seemed the most appealing and knocks on it. If no-one was home he’d just go to another door.