I tried a few tools like PC Inspector FILE RECOVERY, Undelete tools, using the USB-stick on another computer, DOS environment, show hidden folders etc.
If a file or a folder is hidden, you have no trace it ever existed. The only way to trace it, is to compare the free space on a hdd or a stick and compare it to the total amount of bits and bytes of the files stored there.
If you have a 4GB stick, hide 2 GB, and store 20 files with 500kb alltogether, the properties of the stick will show used amount of 2,5 gb.
Apart from that, I (personally) have not found any way to make files visible again.