I highly, HIGHLY doubt that the truck was ever at any point tied to a gameplay feature. There would be some evidence left in the game of such a feature.
In effect, all it is is a piece of scenery. Considering it's nowhere else in the game, it's probably just a graphic they decided to cut at some point in development but forgot to remove from one map, and no one noticed before release because you couldn't get to that area in playtesting.
The fact is, ALL games have unused sprites and assets. If it weren't for the mistake of forgetting to remove it from one instance, there would be no mystery around it, like there's no mystery around the various other unused objects in it and other games.
