Files found but they're not there?

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.