I once got an alert in the quarantine manager and after some digging it turned out to be an spam email (in the spam folder) of Mac Mail. It kept coming back when I accessed the email.
You should clear the alert and rescan - no point spending a lot of time looking for something that isn't there.
If the alert comes back and you cannot see the source I'd run the SDU log gathering tool (Mac version) on your computer and raise a ticket with Support.
SDU: Sophos Diagnostic Utility (SDU): how to download and install
Raise a ticket (ensure you include the SDU output): Support query form
I once got an alert in the quarantine manager and after some digging it turned out to be an spam email (in the spam folder) of Mac Mail. It kept coming back when I accessed the email.
You should clear the alert and rescan - no point spending a lot of time looking for something that isn't there.
If the alert comes back and you cannot see the source I'd run the SDU log gathering tool (Mac version) on your computer and raise a ticket with Support.
SDU: Sophos Diagnostic Utility (SDU): how to download and install
Raise a ticket (ensure you include the SDU output): Support query form