There isn't any proof because that kind of thing simply can't be kept a secret. I brought up before that Jacques Vallee had fed all the landing data into a computer (he is a computer scientist after all) and calculated how many landings there have to be to make this data comprehensible. The conclusion is that most take place at about 3:00 am in remote areas and are not seen by anyone. It works out to 150,000 landings a year meaning about 3 million over a 20-year span. So the evidence should be overwhelming if this were the case. That aside from the fact that 3 million landings in twoscore years is absurd. Ain't happenin'.