avast scans and alerts to infection, it doesn't move it to any folder in your email client, so it 'should' be in your inbox.
If an email is considered infected then avast alerts, your image and previous post basically confirm there was no infection detected or you would have known (the avast alert is both audible and visual and wakes the dead) and the Last infected: field would have had the email detail in there. So avast doesn't move it anywhere and it 'should be in the inbox,' as I mentioned earlier.
Some email clients have anti-spam incorporated and also direct spam to a specific folder (MS Outlook for one), I don't use Thunderbird, but it has the ability to use add-ons and I don't know if you have any that may deal with spam ?
Other than that I don't really know where you might look. Other than to send out an email to your self and see what happens to that.