If you are sitting in a vehicle as a passenger and tab out to some other activities like for example a web browser (DayZ losing focus), the vehicles engine sound decreases very fast.
It seems like the character (or just the "ears") is virtually staying at the position where you left game.
You can still listen to every other sound made around that last position.
The same thing happens in ArmA 3 if you open a map while in a car or helicopter.