I was in Rachel, NV about 10 years ago, and went to the Little A' Le' Inn for a mid-morning coffee & piece of pie and started looking at the photos of "extra terrestrial" thingys that people sent to the restaurant.

One of the photos was a Polaroid of an empty field. A note describing the photo said that the person could see a flying saucer in the field, but the photo didn't show it - which was positive proof that what he saw was a real flying saucer....Okay .... a photo of nothing proves that something was there. Hmmmmmm......