Tough question. It could be any one of those scenarios that you mentioned or it could be that the "trigger area", for lack of a better word, is larger than the area of the picture that the camera takes. For example, a deer could pass just to one side or the other of the camera and trip the sensor yet not be in the field of view. I seem to remember someone else posting about this early last fall but I could be mistaken.

I guess the "remedy" would be to reposition the cam a few times to see if that is the case or if it is something else. I prefer the "short range" setting myself as I know that whatever trips the cam is going to be in the picture.