If there was a helicopter in the area, it might not be seen, first of all it would probably fly during the day and not at night but if it flew at night, it doesn't necessarily need lights, could just use an infrared (night vision) camera.
But surely the Mansi would hear it and talk about that in their testimonies? Instead of saying there was nothing out of the ordinary, they would note a helicopter flying to the middle of nowhere for no apparent reason