Did his backpack have legs?
When he dropped it in the park when the guys were chasing him, he dropped it at the bottom of the stairs that Juice chased him down, and it was somewhat covered in leaves/foliage and stuff. However, later on when the car and Preston are racing towards the backpack in the park, and the jogger picks up and begins looking in it, the backpack is in a large cleared out area of the park NOWHERE near the stairs where he dropped the bag initially.
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Me First and the Gimme Gimmes (<-- best band in the world.)