I know one way it could be done. It would depend on the volume of the truck, but you could either pump air into the truck to displace volume of the stolen booze or by lowering the pressure within the tank, it would allow the alcohol to expand.
That's a real engineerial solution. The problem with it is that the engineer can access the tank during down time only, when the tank is meant to be empty.
When the filling opening is open before a shift the pressure inside becomes equal to the atmospheric.