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They don’t actually scan the site for viruses, they check other blacklists.

@ demarco
The detection is URL:MAL, which may not be directly on your site. This could be a 3rd party link present on your site that goes to a blacklisted/infected site. It could also be that whomever your site is hosted with has lots of sites hosted on the same IP address. if some or many of them become infected then it could be that the IP is blocked.