And since this is in the Dashboard, I'll just post this useful contribution from a BA staffer in another thread:
Originally Posted by
sammyh25
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.
And the answer to the last point - how long in T5 before it goes to T3 appears to be 2 days or so.