On our servers the forests on the Western and Northern edges of the map are common areas to store vehicles. Of course every now and then we find vehicles hidden in small forests in the middle of the map, but at least bigger vehicles (Ural, V3S, buses) tend to find their way to the thick forests in the edges. Running with a helicopter and looking down is kinda fun since most of the vehicles show up like sore thumbs. Finding helicopters on foot would be easy, since they have to be on a clearing with flat ground so they don't ski down the hill :-)