I'd remove add-on anti-virus tools — the scanners themselves can tend to be problematic — and — as you've fond — they don't find various sorts of problems.
Usual path for removing this sort of stuff is accessed via Safari Preferences, and removing the browser extensions that you didn't intentionally install. (Safari > Preferences... > Extensions.
The tool formerly known as Adware Medic can probably remove this, or you can install Etrecheck and get a listing of what add-ons are installed.
Usual source of this dreck tends to be from freebie ad-ons, system optimizers or cleaning tools, bogus anti-malware tools, performance-enhancing tools, video players, "cracked" software tools and downloads from download sites, etc. It's increasingly common to find software sourced from somewhere other than the original are compromised, unfortunately.