It may be necessary to use the DLP Diag tool on the specific endpoint to retrieve data about the USB key connected. If you go into the DLP installation directory, there is a DLP Diag tool in there. If you open that tool (note, this will require a DLP Bypass key generated from ePO), there's a device tab that you can use to get details about any device connected to the system. I believe that, if you can't find the serial you need elsewhere, you can find it in that tool.