There are a few secrets that I was able to discover here, nothing spoiler wise, feel free to read :]
1) If you know on the window a few times at the beginning of the game, a cat meowing can be heard.
2) If you check the non-door doors a few times at the end of the game, Camiel has additional dialogue.
3) The ghost's age is 45, when you answer that you get the "Existential Crisis" item.