I knew a guy who owned exactly that kind of business. Filling private ATM machines. They rely on being ultra-discrete, if not secretive. They can't afford armoured cars and Fort Knox security so they have to rely on keeping a low profile and spy-like "tradecraft" to do their work. It is extremely hard for them to hire workers to drive around with a few hundred thousand buck in a sedan and stay safe. Background checks are never adequate and once your operation is burned by anyone (think janitor's girlfriend) it is only a matter of time until some low-lifes like these guys plan a heist.
No lock on the front door is going to stop an inside job but yeah, this operation was staggeringly insecure. As you said, not even a lock on the front door.
The stress and responsibility to the safety his couriers/guards was too much for him and he sold the business.