Some call it bandit camping. Some call it guerrilla camping. I call it stealth camping but it's all basically the same: sleeping overnight anywhere you don't have permission to do so. It's often illegal and sometimes dangerous but it can also be an absolute necessity and even kinda fun. The very name refers to a kind of self-fulfilling prophecy, as in, "if you got caught, you must not have been stealth camping". To anyone who says "I've never ever camped anywhere without permission!", I would ask "How long you been on the trail?"
I first heard the term from my brother while we were planning a long motorcycle trip. He called it guerrilla camping and described it as "camping in the corner of a farmer's field". I started doing it myself in the Great Smoky Mountains of Tennessee. That is one heavily regulated national park and if you're thru-hiking, you're only allowed to stay in the shelters provided. Unless they're full. Full of soaking wet rookies and snoring old men. And they're way off the trail. And it's late so you'll wake up everyone in the shelter. In those situations, you find a good spot and you play by the rules. Here are the rules:
1) Set UP after sunDOWN.
2) Break DOWN before sunUP.
3) Leave no trace.
4) If anyone says anything, just move.
Ideally, it works like this: You hike into a small town late on a Sunday evening to get your re-supply box from the post office that opens at 9am the next morning. Before it gets dark, spend some time asking around. Often, a kind soul will give you permission to stay on their property or they'll know of a city park or a church yard that is safe. If not, start looking. You want somewhere discreet. Somewhere that even the faint light from inside your tent will not attract attention. A place you will only have to come and go from once. And, if you're like me, somewhere that offers the most challenge - the most risk. More on that in a minute. If possible, you scope out your spot while it's still light outside. Is it as discreet as you first thought? After dark, you slip off the sidewalk and get to your secret spot. Set up your tent quietly and quickly and climb inside. As soon as you wake in the morning, reverse the process and make your escape. Hit the post office as planned and get back to the trail.
The rules, as above, are written in stone. 'Leave no trace' means no trash, no fire, no noise... nothing. There should be no sign that you were ever even there. And if anyone is uncomfortable, for any reason, with you staying where you're staying then you have to move. Even if it's the middle of the night. Even if you don't think they are the property owner. Doesn't matter. Offer an apology and pack your shit up - apparently you weren't as 'stealth' as you thought. And you shouldn't talk about where you're camping (especially to other hikers) until after you do it. I'm always up for sharing a good campsite but stealth camping is best done solo.
I've stealthed in big cities and small towns, in national parks and farmer's fields. I get a weird thrill out of finding ever-increasingly dangerous spots. I've stayed behind a county sheriff's office, in a cemetery, near an airport runway and at the foot of a giant wind turbine generator. I camped near a neighborhood that was made up entirely of NPS Rangers - the very heart of the beast! I once found a spot behind a library that had power outlets, a water spigot, a strong wifi signal and nice little privacy fence. That's as good as a Holiday Inn, to me.