You have all the secrets I know of. I still need to map out the game to see if the drawings on walls are for something. Something fun to do after you finish the game is look at the unused files and graphics. There is a Rez person in a cameo folder, and a photo person in a camera folder. I can't find them anywhere in the game so its kinda cool to see something other people might have never seen.
I don't know, I tried to find anything strange in the code. I did find a door component that was noted out, it was supposed to take you to a room with an id of Photo Booth. Though I can't find the room, and when I un-note it an error occurs. Though Rez might be somewhere, and room 15 is not noted. So room 15 may be accessible. I will continue to search for anything cool.
You might not know this, but there used to be a page where the developers had a dictionary of every single symbol within the game. Every picture had a full written explanation. Sadly the page seems to be down, or I can't find it. It might have been useful since the drawing on the map in town has a 3 symbol message from the howl person.