// Get the destination.
_dest = (_this select 3) select 0;
// Get a random direction
_dir = random 359;
// Move the person 15 meters away from the destination (in the direction of _dir)
player SetPos [(getMarkerPos _dest select 0)-10*sin(_dir),(getMarkerPos _dest select 1)-10*cos(_dir),+15];
Note the 3rd parameter I added to the last line, it defines the height of the setpos command (in this case 15 meters). Eventually you must adjust it up or down.