Yes, Symantec can scan flash drives once it is opened. I haven't had any autorun.inf virus so I didn't test it out. But when checking the reports from various users in my network, Symantec does detect any malware found in the root directory of any removable storage inserted. If the virus is in a folder, it will be detected once the folder is opened. I guess Symantec decides to go this path for the convenience of users. Scanning a flash drive when it's used on the PC would slow down your access to the drive. And as all users want it fast, they'd probably cancel it anyway.