You didn't say if in that thorough scan you selected the scan Archives, if not it wouldn't have been scanned as they are inert.
Archive (zip, rar, etc.) files are by their nature are inert, you need to extract the files and then you have to run them to be a threat. Long before that happens avast's Standard Shield should have scanned them and before an executable is run that is scanned.
You can try a right click explorer context menu (ashQuick.exe) scan which is the most thorough scan it will scan everything selected and use all unpackers for archives and see if it is detected.
Off the link that you gave is this:
Known Problems
False Alert Problems: Some Antivirus programs detect Mail PassView utility as infected with Trojan/Virus.
So it may be that avast is treating it correctly whilst Threatfire isn't