I have seen folder names ending with sub or mosaic in the s50.
What are these?
Folder types in S50
If you turn on the option to save a FIT file for every 10-second exposure you get the *sub folders, this is where all those files are stored. I assume the *mosaic folders play a similar role for when you try a mosaic capture, but since that feature does not work for me, I never payed them any attention.