Most of the scents match with the visuals of the place. It is old and rotten. The dead basilisk does seem to be the only of its kind, and there are no other scents of living beings that might have passed here recently.
In the room, where the basilisk was hiding, part of the outer wall has collapsed, undoubtedly, the basilisk has come in here that way.
Luckily, the exposure doesn't seem to have ruined the books in this room, so they are still intact, at least as far as that is possible after half a century.