Heres a few secrets that I found
Right before you go into the room with the 3 water slides, theres a series of signals tapes, all of which hold the same signal "unnamed". Collecting these and taking them down the slides, you can play and process them. Here are their levels in order.
For the second and final secret I found(that wasnt already posted) you're going to need to use the Debug Teleport button(for me its [Num Pad .], you can check at the bottom of the keybinds). This will take you to a secret room in the tutorial that is completely isolated. It starts off in almost a lobby before entering a section that somewhat resembles a church imo, but theres likely no significance.
EDIT: It should be noted one of the fountains in the "church" isnt running, this may have some significance but Im not betting yet. No screenshots unfortunately, it was a detail I remember but didnt capture.
Anyways, thats all the new things Ive found so far. May post later, but its unlikely that Ill uncover anything relatively new.