Hmm that is weird I will have a think about it ..
For the USB sticks this is the best protection
Download
MCShield to your desktop and install
It will initially run a scan and show the result as a toaster by the system clock
Then in the control centre select scanner and tick
unhide items on flash drives
Plug in the drive and McShield will start a scan
Then get the log which will be located under the logs tab on the main page
And post that