...* If was dark and a gravel road, is it possible he was accidently hit? A driver or passenger might have taken his body and his gun. I'm guessing they weren't testing for tracks or accident recon then.
We know someone's tracks went to the water, but I doubt they beamed up to the mothership. So if they stopped there did they continue downstream or did they 180 and return? ...
A person on a bicycle, on an unlighted country road could have been hit by a motor vehicle, and that might have been considered by investigators.
Besides potential evidence of such a scenario, like: skid marks, blood or debris on the road, or visible collision damage to the bicycle, - one would have to question how Dickie, all of his clothing, and his shot gun disappeared permanently from the scene.
The fact that tracking dogs followed two separate tracks FROM the bicycle would tend to indicate that it was placed in the ditch by someone (Not necessarily Dickie) who walked, across the field, away from it.
IF --- a tracking dog had been given Dickie's actual scent to start with (from his pillow, clothing, a personal item like a wallet, etc.) it might be concluded that the scent the dog followed away from that bicycle was left by Dickie.
HOWEVER, we have no evidence of that being the case (that the dogs were given his specific scent). More likely, the dogs were simply started at the bicycle and allowed to pick up on the scent of anyone who walked away from the bike.
This could have been the person who placed the bicycle there, and/or one or two of the previous day's searchers.
The story that the dogs followed two separate tracks would tend to support the above theory.
In my experience with tracking dogs, they will always proceed from a starting point in the direction that the track layer traveled. This is something that they do instinctively. Reverse tracking is something that is not natural for them and would require a lot of special training.
Unfortunately, we do not know the records of those tracking dogs and their handler(s), or the specifics of their work that day.
Another thing we do not know is what condition the bicycle was in. Was there any evidence of collision damage? Was there any damage from a firearm? Were there any mechanical problems with the bike such as a flat tire, slipped chain etc. which might have caused Dickie to leave it where it was found?
If his intention was to go goose hunting, why would he leave his lunch and a box of shotgun shells behind?
Were those other items securely attached with the bicycle - or scattered about?