I realized that some walls in the crypt keep dark after casting the illuminate spell.

In the thread "What a dumb thing to do" RavenW writes:
>

Does this mean that these dark walls are always secret doors and can be opened somehow?

2nd question:
Is there a possibility to notice that my spirit eye spell stopped working (after 5 minutes or so)?

thanks.

Not always! In RavenW's case the room was not particularly dark and the secret door was a slightly different color from the wall areas next to it. You can try finding an opening for what you suspect is a secret door, but don't waste too much time on it. Generally all the secret doors are fairly easy to find and I'd hate for you to spend hours trying to open something that is just dark and not a door at all!
Lots of players seem to really like the spirit eye spell. I never bothered to use it. I believe that the patch does correct some problems with the spirit eye spell so that it works more as it should. Without it, spirit eye would not always reveal important switches and other things of note.
Wyv

In the crypt there are indeed some secret doors in the north east. Look behind some crates for a button to open it.
I usually notice spirit eye ended when I don't see any red dots from the vermin crawling around anymore.