How could this work? I would expect this method to yield a bunch of matches corresponding to every version of the file that was once saved and continues to live on the disk. Unless you happen to have a string that only existed in the last version or so, but that's hard to come by...

I once did recovery of this kind for a friend (using, I think, photorec or extundelete, not grep) and the hardest part by far was piecing together the "right" version of the files from all matching versions that were recovered from disk.