[shintakezou]

«From the police POV, a child was left alone in a vehicle that was later driven off by someone else».

No "someone else", but the very same woman: leaving the car, returning back to the car. The same woman, not someone else. Unless the good samaritan has deceptively shown just the last part and told the police he (or she) saw a different person leaving the car with the child inside.

The issue here is not car theft, driving license not matching the car owner, or whatever. It was instead a good samaritan evaluating as criminal/dangerous the fact that a mom left his son alone in the car for few minutes. And decided (when she came back) to call the police to...? Check if the child was fine? Whatever.

It takes few minutes to check if the car was stolen and/or the owner knows that someone else is driving it.