Card Missing From Dummy
A card from dummy was inadvertently hidden under the bidding box and this wasn't noticed by anybody until quite near the end of the hand. The card is returned to dummy but should have been played earlier. As dummy cannot revoke, what (if any) rectification or adjustment to the score should be made.
Comments
It sounds as though dummy has revoked, but what you probably mean is that there is no automatic trick adjustment when the revoke was made in failing to play a card from dummy. You can however adjust using Law 64C if the revoke has caused actual damage.
You can also adjust using Law 12A1 if damage has been caused by the failure to display dummy correctly, even if no revoke has taken place.
It would help if people were taught that "Dummy cannot revoke" is just wrong: of course dummy can revoke.
Although of course dummy can now ensure that dummy follows suit - previously such a statement was presumably dummy suggesting a line of play and vetoed.
Somewhere a long time ago it was pointed out that if declarer asks for a card from dummy which constituted a revoke the law suggested dummy should just play it without comment. The WBFLC said that dummy should not play the card but gave no further advice. The ensuing scenario this could create was very amusing.