I've long been in the habit of running an online scan a few times a month to "doublecheck" my AV for this reason, and I use different ones in rotation.

F-Secure Support pages: F-Secure Online Virus Scanner
Free online antivirus. Download ActiveScan and clean your PC. Panda Security
Free Virus Scan - Kaspersky Lab
Trend Micro HouseCall - Free Online Virus and Spyware Scan

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