There is no lock on the door to the upper deck. SIA Cargo often have Horse-grooms travelling on the upper deck, using the 6 seats. Just before closing the main entry door 1 L, the agent responsible for security is supposed to check all upper deck areas before presenting the completed form to the Captain for his signature. Possibly, the stowaway hid behind a cargo pallet before climbing the stairs to the seats. It should be of grave concern to all freighter operators as it is a weak area in security. A locked door from the flight deck to the upper deck is not really feasible as it isolates the crew from "unattended -but-properly-briefed-occupants" in the seats and the only toilet and galley is outside the main escape route (upper-deck door on the right hand side).However, it would be possible and easy to lock the door at the top of the stairs, preventing access from any stowaways hidden in the main deck cargo. Of course, it would cost money!