If NASA is hiding intelligent alien life, what would be their reasons?
Bear in mind that, to an alien, America isn't the be-all and end-all.
If extraterrestrials visited Earth, they'd have turned up all over the place, not within the exclusive geopolitical boundaries of one nation. So it wouldn't just be the NASA/CIA axis hiding aliens, it would be a global conspiracy of space and spy agencies.
Likely? Not really.
Oh, I'd love to see evidence of extraterrestrial visits, particularly benign visits. But if they happen, we'll all know-I'm sure of it. These days, with smartphone cameras and global communications, it would be just about impossible to keep the lid down on that kind of struggling animal.
(Footnote In case anyone wonders: no-I haven't worked at NASA. As a researcher and alpha geek, I've processed NASA data since the days when I had 1″ magtapes couriered over to me from NASA Ames. These days, it's easier: I get them to take images of areas of interest, particularly the HiRISE camera on the MRO, and pick them up over the net.)