Here's a list of all the secrets I have managed to find so far:
spoiler:1. Upstairs, in the "library" there is a trigger on the right side of the fireplace base, just by the floor. It opens the metal grate in the fireplace. (Thanks to ShadowWolf421 and Dino; I was wondering why there was blood under the grate :-))
2. In the guard shack outside the entrance. The trigger is behind the bunk to the left of the fireplace, by the floor. (Thanks, Dafydd!)
3. In one of the servants' rooms (two bunks, two lockers), frob a crayman's head trophy on the wall. It opens a secret door in the opposite wall.
4. In the bathroom there is a very small lever under the table, on the left side of its support. This opens a hidden compartment behind the potted plant.
5. In Hendersen's room (the one with the blood-stained carpet) there is a hidden switch somewhere in the table. I couldn't find its exact location but I managed to frob it: Get as close to the table as possible and face the golden thingie on it. Then look down (about 45°) and click. It's tricky but it can be done.
6. The last secret is in a small basement room. It's the one with the letter from Larson lying on a table and
a painting of a skull hanging directly above it. The trigger is somewhere behind the left window curtain. I didn't see it but "using" the bottom right corner of that curtain did the trick. I was very lucky indeed to discover that one!
7. In the hall with the Christmas tree, use the statue near the huge banner to make it move aside and reveal something very useful.
Some of these apparently don't count as secrets as there are only five of them. (Probably 1. & 5. as they are plot-related).
Enjoy this great mission!
LOTHAR