@ebinkerd
I understand exactly what you are saying, and I don't disagree with any of that. I've been working with electronics for a few decades.
It's not unusual to see various 'grounds' at slightly different voltages from each other. My point was that this should be corrected when it is observed. Any ground point needs to be as close as possible to 0v relative to the battery negative terminal, be it the alternator, a light bulb, or a sensor. I have corrected quite a few issues on various cars by cleaning up ground wires, and when necessary running additional grounding straps to improve the circuit.
If something is marked as a ground on the circuit diagram, then it needs to be as close as possible to 0v. Otherwise it would be marked differently on the diagram, speaker + and speaker - for example.
When you have different ground points at different voltage potentials, you are going to have issues.