I have never worked in Law Enforcement, I retired as an electrical lineman/serviceman. A lot of my job required that the engine in my truck stay running. The engine powered the hydraulic system that worked the boom. There was a lot of times that I had to park in front of a customers house and go around to the back yard. Whether to turn a meter on or off or to climb a pole and fix a light. Whenever I was assigned a new truck I would immediately go and have a couple of extra keys made. I left one in the ignition with the truck running and carried one on my person. For safety I would lock the truck anytime I was in the air. Ya don't want the truck stolen when you're 40 feet in the air! Later on we got computers in the trucks and if not kept cool they would overheat and fade out. Sometimes it would take 30 minutes for them to cool down and come back on. My orders were on the computer and I didn't have time in my schedule to wait 30 minutes. Our trucks also had all kinds of flashing lights and strobes that would drain a battery. I kinda sidetracked the thread but there are reasons to leave a vehicle running.