The very first room of the tutorial has as a secret tile that is effectively invisible, and the tutorial for fixing servers gives you 3 of the 4 numbers for a keypad (not in order, I believe). I have no idea how you are supposed to figure these 2 things out because the rest of the secrets aren't too hard or are just minor time sinks to figure out with some thinking and snooping around. I assume you'll also want the fourth number instead of trial and erroring roughly 400 number combinations, so I'll tell you that it's seven.

Additionally, the vent cover is red herring... but there is a vent. Anything you can break open *usually* makes a different sound and always makes debris particle effects (same as the normal game)