The bible is very clear that we could entertain angels unaware, we see from scripture angels have visited many in the bible, we also see from scripture its highly likely that anyone who will inherit salvation will encounter one of these ministering spirits either knowingly or unknowingly (see Hebrews 1:14)
In this truth of scripture do you see any clear photographs of these encounters the bible confirms will happen............much the same way when an evil angel appears and ministers to people such as Joseph Smith in leading men from the truth none of the tangable evidence is ever allowed to be held, such as the golden plates, or the supernatural glasses that allowed him to read them and interpret them, My guess is there will never be conclusive evidence that angels exist whether good or evil,
my personal belief is UFO's are demonic and possibly technology used to bring those in power to believe that they are some sort of other life form, when in reality its just another satanic trick to make men believe in other gods,