There have been threads about this before.
Apparently the only 'keys' you get with your shiny new 747-400 are for the cockpit and come with a nice 'Boeing' keyring. These keyrings are highly prized and tend to get half-inched pretty quickly.
You can't really 'lock' an airliner as such, and you'll notice airliners parked up at museums have padlocks on the doors. Quite who is going to hot-wire a 40-year old VC10 and fly it away I don't know!