Agreed. This is my thinking(basically what you said)....in an area where a homicide victim is recovered, I would expect LE to collect EVERYTHING which doesn't naturally belong. Since this happens to be a fairly remote area which obviously(by what appear to be worn footpaths in videos made by JM and GH) gets some amount of traffic, and my own experience, having grown up near a similar creek that was a local hangout...I would assume that people maybe go here to play in the creek, maybe sneak the occasional cigarette or beer, and teens may even have sex in the area from time to time. It wouldn't be uncommon for kids, teens, or young adults, to discard a pair of cheap shoes(if they were soaked), or underwear(after maybe taking a dip in the deeper parts of the creek), cigarette butts, socks, etc. Anything like that which may have been found within a specific radius of the crime scene most likely would have been bagged and tagged, and may or may not have anything at all to do with the crime.