Files that can't be scanned are just that, not an indication they are suspicious/infected, just unable to be scanned.So you shouldn't be sending files to the chest just because they can't be scanned.
See
http://forum.avast.com/index.php?topic=35347.msg297170#msg297170 this topic for more information on why files can't be scanned.
I would expect S&D to protect its recovery data, I would also expect a Quarantine (yahoo in this case) to also protect what is quarantined.
Move to the chest is quarantine:
In avasts case files in the chest have the name changed and are encrypted.
If you check the chest location from outside with explorer C:\Program Files\Alwil Software\Avast4\DATA\chest all you will see are the names allocated by avast not the original file name. So even if you knew it was there you wouldn't know the name assigned, so it couldn't be run from the outside.
When you open the chest you then see the different sections the only one you are concerned with is the Infected Files section, then you can see what the original file name, but only avast can access the chest.
There is no rush to delete anything from the chest, a protected area where it can do no harm. Anything that you send to the chest you should leave there for a few weeks. If after that time you have suffered no adverse effects from moving these to the chest, scan them again (inside the chest) and if they are still detected as viruses, delete them.
