Summary of NFPA 58 requirements for propane transport in vehicles (excluding RVs) for private use:
Cylinders shall be determined to be leak free before being loaded.
Not more than 420 pounds in an open truck or in an enclosed vehicle with separate passenger, cargo, and engine compartments (this would include in a pickup cap I think)
Not more and 90 lbs and not more than 2 cylinders in an enclosed vehicle like a passenger car or SUV
Must be in DOT cylinders
Must be placed on a horizontal surface or on racks
Shall be fastened in position to minimize the possibility of movement, tipping, and physical damage.
Cylinders less than 45 lbs propane can be in any position for transportation (but for storage have to be upright if > 1lb)
Cylinders above 45 lbs must be upright.
Nothing mentioned about locking or unlocking the vehicle
So if the state/county/city has adopted the NFPA 58 standard (NC has) then they can cite you for not meeting any of the above, but not for other things like just having the propane back there.
Looks like CA has its own standards for propane in addition to NFPA 58. I didn't read through the CA standard since I no longer live in the PRC (Peoples' Republic of CA, don't worry, just a little gentle humor for the great state that brought Prop 65 warnings to all the rest of us
). Here it is if anyone in the Golden State wants some light bedtime reading.
https://up.codes/viewer/california/ca-fire-code-2016/chapter/61/liquefied-petroleum-gases#61