Thank you all for your answer. Azcavalier, I think my BF is not so 'heavy' as yours was. In fact I did some portraits and it seemed ok, even landscapes seem ok...
I am posting some photos I took for checking the issue. I think I will also check with the focus chart later.
So, first one is the complete picture, no cropping. The focus point was on the white book (Vicente Segrelles). f2.8, 1/250, ISO 200, hand-held. The book on the right was about 6 inches behind.
And here it is a 100% crop of the focus point:
As you can see, it is out of focus. Next one is a 100% crop of the book on the right, which was a few inches behind:
This one is quite on focus.
I had to repeat the test 4 times to get a good one (focus on Vicente Segrelles), or at least a not so out-of-focus pictures on the white book.
Maybe the problem is based on what Marc says ('the camera choosing a different subject to focus on than the one you intended').
I have some walking around pictures with a similar problem, in fact it is more obvious when shooting close distances and wide open.
Ok, thanks for your comments.