Andyross
While Comcast says it should "be immediately removed from your account," I guess it would be wise to wait until after this weekend is over, before checking again to see if the old box is still shown on your account or not.
I know the newer boxes have bar codes that can be quickly scanned into their system, but older boxes may not, and have to have their serial numbers entered manually.
Not sure then if Comcast's system would flag an invalid number being entered or not, but that could be one possible reason why an old box might still show up after it has been returned.