When a USB drive is plugged in, SEP will not automatically scan it. Only auto-protect will catch it if the virus tries to activate.
Please submit the file sample to Symantec to see if they detect it at all:
https://www.symantec.com/security_response/submitsamples.jsp
You can also kick off a threat analysis scan using symhelp:
TECH215519: 'How to run the Threat Analysis Scan in Symantec Help (SymHelp)'