Solved! How Did Antivirus Miss This?

The bad guys are constantly evolving malware and ransomware to get past security software. As a result, our Threat Labs and developers are constantly working on technologies that help detect unknown or never-before-seen malware.

Two examples:

CyberCapture - analysis of unknown downloaded files, and isolating them so they can't do damage. They're first analyzed by our AI/machine learning, and if a detection cannot be made, it's sent to our Threat Labs for further analysis. Source: https/blog.avast.com/an-in-depth-look-at-the-technology-behind-cybercapture

Behavior Shield - real-time process monitoring to detect malicious activity on the fly. Source: https/blog.avast.com/behavior-shield-our-newest-behavioral-analysis-technology

Where signatures come in is speeding up detections for known threats; an "unknown" threat can go to "known" very quickly with a system of layered protection that starts with pro-active detection.

To answer your initial question, it is possible that if the malware was "dropped" by a malicious process, simply opening the item could have compromised you, but it depends on the exact strain/variety that you were infected by.

On a side note, if it's something you're curious about we have a feature in certain versions of Avast that can "lock" files so nothing can edit or change them without your permission (Ransomware Shield) as an extra layer of assurance.

I hope all of this helps and gives some context for you! Stay safe