It seems like Casey panicked and then disposed of her dead child along with everything else she had in the trunk. She had to have had a "better plan" as to how she was going to get rid of everything. The body was only a few feet from the side of the road... even an idiot could figure out it would be discovered eventually. I'm guessing that poor Caylee was in that trunk when George tried to go in to get the gas cans, and Casey freaked and took off and dumped everything. Or maybe when she ran out of gas on the side of the road she realized that she couldn't leave her car with a rotting corpse in the trunk, and quickly tossed everything in the woods before calling Tony for a ride. For someone that put a lot of planning into their child's murder, the way she disposed of the body was too sloppy to have been part of that plan.
Luckily the syringe was protected inside of that bottle, and hopefully they can test it to see what was inside. Had the syringe been dumped elsewhere, chances are it would never have been discovered, or connected to this case. This might be the only evidence they have to show manner of death.
All my opinion of course.