IMO, assuming an accident, one of the possibilities is that vultures got hold of the remains at a certain moment. There are many vultures in the Pyrenees (3 types: Vulture, Griffon vulture, Bearded Vulture). If they did, chances of finding an entire skeleton or many bones are small. Don't know enough about those huge birds to say with certainty that this may have happened between Monday and Thursday when the searches started, but it could be an explanation why nothing was found when the area was searched at the time.
This is all speculation.