On my thumb drive, when Recuva starts scanning, it says there are 252 files found. At the end of the scan there are no files in the list. When I rescan immediately afterward, it still says there are 252 files found. What and where are these 252 files??
This indicates that files are being filtered out by Recuva; this is done because there are many kinds of files that either most users would not want to recover (e.g. system files; those are better recovered by reinstalling the OS) or would be pointless to recover (files containing 0 bytes of data, for instance).
This can be controlled in the Options > Actions section of Recuva (you may need to Switch to Advanced Mode to see the options button) as described here: https://support.piriform.com/hc/en-us/articles/360048410512
In short, checking the first three 'Scanning' Actions will prevent Recuva from filtering out any files.