I think you should get the luggage returned to Madrid passanger ASAP.

It could have necessery items in the baggage like jewelry, relatives ashes, who knows... Sure we know people shouldn't put such items in their luggage but infrequent flyers may not have considered the outcome. The passanger concerned may be drowning out his/her sorrows on sangria now whilst having to wait till next week!

Although BA's cavalier attitude towards the madrid passanger seems unorthodox.