An item that was detected and recorded in Quarantine Manager subsequently has the threat entry removed.
This occurs when the detected file is stored within an encrypted sparse bundle. When the bundle is unmounted, Quarantine Manager removes the threat entry when it subsequently checks it.
Known to apply to the following Sophos product(s) and version(s)
Sophos Anti-Virus for Mac OS X
All supported operating systems
Mac OS X 10.5+
What To Do
This is a known issue.