[Ancapistani]

You can get some of this feeling from spending a large amount of time in a given area.

I grew up in a house on about five acres in the country, and spent a lot of time outdoors reading. I grew to know the area, the types of animals that lived there, where they tended to move and where they stopped, and learned to recognize the individual animals. Animal behavior alone can tell you quite a bit - have you ever been in the woods and suddenly felt a little uneasy? That's probably because you unconsciously noticed that the background noise changed. Birds and small animals got quiet around you because of the presence of a predator.

[JohnFen]

Indeed. I've been backpacking in the wilderness about every month for a few years and, although I rarely encounter anybody when out there, I usually know when a large predator (including human) is getting near because the sounds of the wilderness change.

I can't even really describe what changes about it, but it changes. It's nothing obvious like suddenly everything gets quiet or loud. It's subtle, but unmistakable.