That IS interesting re: S1, so I removed it completely from the system, rebooted into Safe Mode, ran cleanup tools for Avast after an uninstall, rebooted again and reinstalled Avast Business... and saw the same issue. Funny that it only flagged it after the latest restart of my computer, so it feels like possibly a Windows update triggered the issue. Uninstalled, ran cleanup again, and installed Bitdefender Endpoint Security, scanned the computer, no issues. Windows Defender: no issues.
It would be nice if Avast was actually putting a file in quarantine like it says it is, but no matter what I found no files stored there, and the web console had nothing to add to this.