Since we still are in that situation, I can tell you..: after arriving on scene the driver circles the ambulance locking every single door.
No. Just take everything you need with you before locking the door. Our equipment is packed in a way, one provider can take all what a patient may need (since we do single provider first responder calls). It's heavy, though. Stretcher, other lifting aids, and special immobilisation items not included (we don't need them in first responder calls because there would be another ambulance arriving soon).
If we need something more, then we have to remember taking the (single!) key to open the door. Since we have a radio only in the ambulance this is true for every message we want to get over radio. So, in a CPR situation I would prefer the phone.
If they have the key it's no wizardry, just with any other (old) car. Fire department would have other possibilities, but I wouldn't like them to try on our ambulance.
No hidden button, just a key in the trousers of the driver. Or in the side pocket. Or in the jacket. Or there on the patient's side. Or just slipped under the bed. Or...damn!
I want to have a central locking system!