Don't Knock on the Window!

This is from the City College campus:
In case you can't tell, the signs read:

Don't Knock on the Window!
Use the Shepard Hall Main Entrance
Thank you!

I have no clue what the first line has to do with the second line. Were people knocking on the window to get into the building? Do they want you to go in the main entrance in order to knock on the window? What motivated them to put up this sign. I'm certainly stumped.