If you select an item in the Quarantine window and choose to restore it, the item will disappear from the Quarantined files window and will appear as a file excluded from the scan (Quarantine / Files excluded from the scan).
Just as you can exclude items from quarantine, you can also return them to quarantine. To do this, select the checkbox corresponding to the item you want to return, and click Undo exclusion. Then accept the confirmation dialog box.
The item will disappear from the list of exclusions, and will reappear in the quarantine list when it is detected again.