This guy I knew once set a hobo's tent on fire. He insisted it was within his rights, the tent was on his property, and when the firewoman asked "You know you'd be in jail for homicide if anyone had been in that tent, right?" he blinked in utter bewilderment as if antlers had sprouted from her helmet.
Sure enough I got a text message from my homeless client later that night, asking where the fuck was a respectable hobo supposed to go now?
The people who are best at hiding (and I say this with love in my heart) are homeless, drug dealers, hookers, and murderers, in that order, and if you listen to them enough you can build a unofficial map of blindspots, pockets of refuge protected by racism and superstition and lazy service providers that allow someone to survive a few weeks longer.
None of these barriers are physical. You could find them yourself with enough cigarettes and a plausible excuse for roaming a bad neighborhood (bonus if you wear hospital scrubs to pass for Not A Cop). If you're not sure where to begin in your home town, a quick internet search of street intersections where the honky trinity of liquor stores, motels, and car titles loans cluster will do as a starting point.
Walk everywhere. The parking lot, the dumpster, the forest, the fences. If you find trash, follow it. A break in the fence, go through it. A flattened cardboard box, ask the store owner where that sleeper hangs out during the day. If you start to look too nosy, tell them your old roommate got out of jail and last called you from a stranger's cell phone in this area (make up a person but stay vague, "Have you seen him? Skinny, paranoid, wears a baseball cap?").
You won't hit paydirt immediately. The trash may lead to a burned mattress, the fence to a cut-through path connecting apartment dwellers to the bus stop, the cardboard box to an alcoholic who refuses to speak. Keep walking. You are a five-year-old on an Easter Egg hunt where the prize is a potential Elba to exile your own clients should the public shelters or homeless encampments prove too dangerous.
Eventually you will find people. If you're in a public place such as a gas station, grocery store parking lot, or highway exit ramp, they will likely approach you first to panhandle or sell drugs. If you're in a more illicit area (forest, abandoned building, the crawlspace under a bridge), announce your presence from a safe distance with a friendly wave and the offer of menthols. Repeat your invented search party story, ask where a loner seeking privacy might sleep, or at least where he'd stash his possessions.
Most homeless stick together for safety and community. Camping alone means you're either sane enough to realize the dangers of street culture or you're too fucked in the head to manage any activity more complicated than occasional dumpster diving. The sane ones know they have a choice and choose to die in the forest. The fucked ones, fucked by abuse and abandonment, died when they were kids. Either way, they will walk a long slog to not be found.
If you're lucky someone will point you in the direction of a hiding place. If you're very lucky they'll escort you there, at which point you can pump them for information with open-ended questions to gauge the stability of the surrounding area (and when I say "stability" I mean the odds that anyone will care that folks are squatting). Do cops sweep here often? Anyone planning to develop the property? Seen the roof blow off a meth lab lately?
If you drop a story involving horrible ways to die (locked in a car trunk and set on fire is common enough) your escort will usually reciprocate with some of his own. He might warn you away from certain buildings haunted by suicides or overdoses, or gesture to lean-tos so far down the train tracks that if you were ever injured you would die trying to walk back to civilization.
He might point out the best spot to hide a body. Dry wells and elevator shafts filled with feral cats. Keep those in mind if your client ticks somebody off.
Repeat this process in every shitty neighborhood in your town so as to have a range of options for a range of desperate clients. Want to stay close to people? The condemned elementary school. Want to stay the fuck away from people? The interstitial green space that separates airports/municipal dumps/prisons from the rest of the city due to noise pollution/water pollution/watching too many episodes of "Oz".
As much of a pain in the ass mapmaking may be, it will buy your client time when their temporary sleep space suddenly becomes inaccessible and they've maintained too small an orbit to secure safehouses in other parts of the city. When the poorly spelled text message comes at two in the morning, "they burnd my tent pleze can u help me", you can navigate them into the safety of the margins.