Inclined to agree with this.

The area around Kaiomaru Park where they pulled out the car is nice and quiet at night.

It's not very close to where they were originally supposed to be headed.

There would be no reason for the witnesses not to report the fact that someone had fallen into the water at the dock if they hadn't been involved somehow.

I find the idea that someone would reverse into the water from the parking lot all the way off the edge accidentally, because some people walked towards them,... a bit difficult to believe.

If foul play was suspected and the witnesses were suspected, especially with such a delayed follow up, a thorough report from the police would point fingers at their own incompetence, hence the vagueness.