If you talk to the Rangers and other park employees, they will tell you it is an escalating problem, that started with firewood and has gone up from there. It can be very region specific, also, from what we have seen.
Sometimes, all you have to do is sit back and watch around noon time. Most of the parks are pretty empty then of vehicles as many have checked out, new folks haven't shown up, and others are out on day trips. There is often someone out picking up anything they can find from "unoccupied" sites, most often firewood. We regularly run into people that say they never buy firewood, because they can always find leftover wood in sites. We also regularly run into people (including us) that have their firewood stolen. Nice coincidence.
If we see someone in a site that is occupied, but the folks are gone, we usually tell them it is occupied, and inform a Ranger about it, especially if we know where they came from.