2 of the 3 sites simply said that the website in question contained a "trojan" with a scary name seemingly made to frighten people away from the site. Googling the name provided basically zero details as to what the problem was. Through googling I found my way to Sucuri who do a free website scan. They then claimed the site was infested with "SEO SPAM". I did more googling and found out that a while back there was a scandal involving some module creators and SEO spam. They had been inserting invisible links into pages containing their modules to increase their SEO. This was the "trojan".
There was no real virus. There was no danger to users computers or even the webhost, or anything except google's ranking. For that reason these scanning sites recommended and were used in such a way that people were prevented or strongly encouraged not to visit the site. All because of a hidden back link.