It definitely looks like the battery was partly submerged in some liquid. Notice the "wave" pattern on the long side. That would seem consistent with a fluid moving back and forth as the car accelerates and decelerates (pointed out by others). But also look at how the residue becomes more concentrated as you move down the side. That seems consistent with a fluid evaporating over time and with the concentration of the residue becoming higher as there is less fluid. At the bottom of the battery compartment is where the material appears thickest.
How much liquid was in the battery when they pulled it out? If you know what volume of fluid the battery contains and the volume of the battery compartment, you can estimate the height at which the fluid would have been on the side of the battery had it had a leak.
What if you scrape some residue onto a wire and put a flame under it? Perhaps it would produce the blue flame associated with sulfur but I leave this to the chemists on the forums. You could also try to see if it dissolves in a small amount of water.