Well, Pokemon could spawn in response to trainers, but then Pokevision etc should have ruined spawns since they act like a crowd of level 1 trainers teleporting around everywhere.
Pokemon spawning has something to do with cellphone density, or presence of trainers. I'm 100% positive.
I work kind of out in the middle of nowhere. There are few pokestops, few houses, and no gyms anywhere near the facility.
However, the facility itself is pretty big. 1200 employees or so, plus several hundred visitors a day.
The kicker is that cellphones are banned from inside the facility for security reasons, so everyone, employees and visitors alike tends to use their cellphones from the exact same spot in the parking lot and nowhere else.
The result of this is that you will be mobbed by 6 to 8 random Pokemon all sitting at the exact same spot in that lot every single day like clockwork. There are a few more wandering but they all cluster in that spot. It's not a nest either, you'll rarely see more than two of the same kind.
I pick up a half dozen when I arrive in the morning and a half dozen when I leave. I've left the app running while going around the perimeter of the building as well as on the access road leading to the facility (it's a half mile) and rarely get a reaction out of it.
The locator apps- poke vision and the github one all show this as well. Complete dead zone with Pokemon randomly clustered in what looks like an empty field.