You might be right.

One thing to consider.... Look at the picture again. Check the "refuse" line under the front door handle. It looks kind of straight and neat. Would it be like that from someone puking? Maybe. I have a funny feeling that line is sort of straight because of splatter coming from the rear towards the front and an object like the step rails or something blocking part of the splatter, therefore creating the almost straight line.

If it is puke... there may not have been much of an attempt to stick the head out very far considering some got on the metal space up between the windows. If that was the case then some got inside the truck. The lack of filth on the front window could indicate a puking also. It could also mean that the window was open at the time of the incident.

Interesting considering the picture is dark. If it was a simple puking would he ask the other users to solve the puzzle?

I have a strange feeling the splatters came from rear toward the front... but I could be wrong. I'm not Henry Lee.