Anything we find on the aircraft after arriving at T5 is taken by the crew member and handed in to the BA Baggage Office in the baggage hall. Whilst there we fill out a form with the flight number, where the item was found, including seat number if known, and a description of the item. The item is then delivered to the Lost Property Office landside, behind domestic arrivals, at some point in the next 24 hours. How long the item stays there before going offsite I don't know.