every ambulance should be locked when the crew is not in it.
older ambulances that do 911 should have kill switches, which allow the doors to be locked, the engine running, and the keys removed while the crew goes inside to assess the patient. older ambulances are more likely to not start, especially in the winter, and you don't want that to happen while you have a sick patient who you just brought out to the ambulance.
just like that, but if you remove the keys, and apply the break, the engine dies, so no one can drive off with the ambulance.
also, driver and passenger should each have their own set of keys, that are kept with each individual at all times. you start the shift with 2 sets, you must return 2 sets at the end of the day.
I keep my work bag, laptop and GPS with me when I am working in the suburbs. I don't want anyone to borrow my personal equipment without my permission.
most city EMS agencies I am aware of have written policies to lock their ambulance when not occupied. maybe the rules are different in the suburbs and the sticks, but I still think every ambulance should be locked (and the power locks work) when not occupied by the crew.