That wasn’t knocking the door casually ! Typically when we see Sheldon knock on doors, he is casual with time to do things properly. In the scene in question, he is even more anxious and rushed than normal, as we can see from his race across the landing and fast talking. OCD people are not robots, and sufficiently serious immediate concerns can on occasion drive the routines from their mind.
In real life ... it’s only the fifth episode ! Give ‘em a chance, they hadn’t thought of it yet :-). (You’ll notice in episode 2 @ 13:40 he doesn’t either, and there he wasn’t even rushed ;) )