File is called “sync.ffs_lock” in the root folder of the Cryptomator drive. As per subject — appears when accessing the vault via Cyberduck interface but doesn’t appear in the (Windows) virtual drive
File itself is (as name suggests) a lock file for “FreeFileSync” which I used to compare the cryptomator vault with unencrypted local folder tree. From timestamp, it is a few days old and totally ok to delete but thought it is very surprising/strange I cannot see using cryptomator apps but can see via Cyberduck