In my area many weekenders stay past the checkout time on Sunday. Most will kind of pick up camp, maybe hook up TV, and continue camping until several hours after checkout. Most times no one comes for the site, but I think the offenders would leave quickly if needed.
Another thing I've noticed at our SPs, which have a checkout time of 4PM, is that the weekenders reserve/pay for Thursday night, even though they have no intention of coming until 1-2 o'clock on Friday. It's only an extra $20 cost, for the convenience of knowing the site will be empty.
Camping is on the up swing, everyone wants to get more leisure time in. They are working around the system to do it.
Jerry