My friend fell victim just now to a scam. He connected to a website in order to live-stream a hockey game. Then a pop-up appeared, telling him that his Mac was infected and he should call a certain number, which he did ... Innocently, he allowed these guys to remote in to his Mac and "do a scan". Then later on he was worried and called me.
I connected to his Mac (using Teamviewer) and installed Malwarebytes, and ran it. It took only two seconds or so to scan, and reported no threats.
Unconvinced, I went to housecall.trendmicro.com and downloaded their scanner. It also took only two seconds.
I am not experienced with Macs. Is it possible that those two products could finish a scan of his system regardless of how much stuff he had on it, in such a short time?
Or is the malware software that I'm sure these guys installed somehow short-circuiting the malware scanning software?