I would like to believe that once a files is securely deleted, it's gone, baby gone.
But there's an option to show securely deleted files. Granted it's just the name... but if the name wasn't deleted with the file, where is it residing? In the MFT or FAT tables?
everything leaves traces no matter how securly it was deleted it just depends on what those traces contain (if the secure deletion was done right probably junk or zeros)
Nergal is correct. Despite all those fancy words (delete, erase, wipe, wash, clean, scrub, etc) the only thing you can do to a disk or flash drive is write to it. Files that Recuva thinks are securely deleted have an entry in the MFT, and that entry points to (what we hope is overwritten) space on the disk. The MFT entry and the disk space will be overwritten with further use.
I don't know how Recuva classifies securely deleted files. It can't be the all ZZZZs name, as some are occasionally listed with the non-securely deleted files. As Recuva and CC are applications writing data there's nothing in NTFS or Windows that says 'This file has been securely deleted.' It's just data, and secure deletion merely a concept.