I've wondered about backpacks before, both at DCHS and at Black's Beach. I usually try to keep my backpack within eyesight, but I can't spend all day babysitting it. One time I put a drink in the the shallow water of the creek to cool off while I played in the springs, and an hour later I saw someone drinking it. That's more understandable - to them it probably seemed abandoned. But considering my backpack has my wallet, keys, phone, and clothes in it, I'd be REALLY screwed if someone took it.
One more note abut my trip I forgot to mention - right as I was leaving (around 1:30 or so), someone was putting up a new slack line. Not sure now 5 days later if it's survived, but there'd been some people here curious about its latest status.