I don't use calibre's Autoadd, but I use a similar feature in a digital asset manager application.
It has options for what to with items Accepted/Rejected — Discard (permanent delete), Remove (send to Recycle bin/Trash can), Move (to another folder).
The default setting for Accepted is Remove. The default setting for Rejected is Move, and the default for the folder name is the <autoadd folder>/Rejected.
BR