In the case of the BA777, if the door had separated straight back without hitting the cabin window the crew would probably have been unaware of it until they landed.
The 777 panel is in an unpressurised area and has no flight deck indication - it is purely a maintenance access panel.
Last week a BA 737 had a tire separate on take-off that knocked off the gear door and damaged the wing and flaps - the crew were totally unaware until told of the incident by ATC - so a lot can happen without the crew knowing.