Thanks for answering.
I knew that wipe free space removes deleted files for good (while recuva allows to recover deleted files because they're not deleted for good!), but hadn't really thought about what would be written over.
But, anyway, not sure if that helps. What I meant is that I do run CCleaner on a daily basis, including the wipe free space routine, and it never said there was such an huge amount of free space to be wiped. Two days ago, I've runned CCleaner and there was no unusual thing. But now, suddenly, there're 40GB of free space to be wiped, even though I couldn't have deleted such huge amount of data in this period. Can't say that this isn't weird!
Updating the situation: I did run CCleaner with the wipe free space routine on all the way, and guess what, apparently nothing happened (if I try to run CCleaner again, it will still want to wipe the same amount of data). Also, I did try to use Recuva to "remove securely some big files", and then when I tried to wipe free space on CCleaner there were more free space to be wiped, which is the opposite of what should happen, as that space would have been wiped by Recuva..