If it got stolen
and you were eating in the truckstop for an hour
or
you took a motel for the night
or
you went home for the weekend
or
you dropped it and botailed home
there is a good chance your cargo is not covered and you will pay
out of your pocket
you must have a specific "unattended vehicle rider"
OOIDA Risk Retention has one and I believe its part of the policy
Lloyds of London does not cover unattended vehicle
Check your policy for exclusions