I live in the middle of farm country and help my neighbors during harvest when I'm not hunting. Even before the rut when the big bucks are most keen will walk right out in a field that you're driving a tractor or combine and feed like nothings going on. You can drive around for hours and watch them not more than 50 yards away. But once you change the RPMs they immediatly go on the alert.
I think the same goes with ATV's. If you are driving at a steady pace and your quad is not obnoxiously loud, you can drive right up on a deer and it won't run until you change the engine sound by slowing down.