Confirm If Website Unsafe/Compromised

Confirm If Website Unsafe/Compromised

Neither are recent, but I've encountered two websites that are on benign subjects but appear to be compromised and/or have triggered/transient pop-ups or redirects that seem malicious. I run websites I'm not familiar with through Virus Total & Norton SafeWeb, and neither detected anything. Is there a reason neither would have picked them up? I reported both to Google Safe Browsing, and they both came back clean. 3 questions:

1. I'm confused, is there a reason that none of the security scans would pick these up (other than my being mistaken about their being malicious)?
2. I've since gotten BitDefender with Webshield, but in light of none of the websites picking anything up, is there something more I should do to ensure safe web searches?
3. I've performed scans with built-in McAfee software as well as Avast & Malwarebytes, and all came back clean, but in cases like this, is there anything else that should be done?

For peace of mind, if anyone knows someway of safely validating these websites, I can provide links in a reply.