I examined the mirror, which produces a chilling response. The problem there is that the mirror is not mentioned in the location description and it is not an object - and it is not even very obvious in the graphic, so it is very easy to assume that there is nothing there and there is no point in examining it. By contrast, the toilet is clearly shown in the graphic but examining it produces no specific response, and the same goes for eg the TV in the room upstairs, which is mentioned in the room description but has no examine message.
I haven't found a hidden room. I've played through to the golden door ending, but the locked metal box is still locked. Trying to unlock it produces the response 'it's locked' where I would expect 'you don't have the key' or something similar instead.
But I don't want to be overly critical: I can see that work has gone into the game and it works well enough. But it could be better, I think, with a bit of a tidy up and a little more description and a few more responses for the player.