Thanks Dallas,
Someone has nicked it for their infinite improbability drive.
It's in a box closed by a bolt.
The truck is vibrating sufficiently to shake the bolt loose - how was the driver feeling after 20 hours of being shaken like a Bond cocktail?
The bolt shakes loose and the capsule escapes via the bolt hole (suspends disbelief).
Then it falls off the truck (Bedford tray-top? - not even a canvas load cover tied down to the side rails?).