So I got home and plugged the stick in and noticed it was pre-partitioned for some fancy security shenanigans (which of course only works on Windows anyways.)
It was in this partition that I noticed the previous owner had stored multiple images.
Huh, I guessed that the store clerks only checked the "storage" partition and not the "security" partition.
Anyways, I opened up the first image. It was a photo of a woman in dark hair, dimly lit, looking very exhausted. Then I noticed the bruises.