I wouldn't extrapolate that to aliens. Too much else would be going on. (Technological advances, invasions, who knows what?) As much as I loved the X-files, Snowden would have outed this secret project. :-)
If you're actually asking whether they're alien spacecraft, well, there's a million dollar prize for anyone who can prove that they are, and it's yet to be successfully claimed:
http://www.csicop.org/si/show/fakers_and_innocents_the_one_m...
But alien spaceships, most unlikely, though it disappoints me to say so. I don't think we live anywhere special in galactic terms, or that our technology or environment is so interesting enough as to attract extraterrestrial attention.
1. There are 2 types of UFOs — the ones we build and ones ‘they’ build. We learned from both crash retrievals and actual “hand-me-downs.” The Government knew and until 1969 took an active hand in the administration of that information. After a 1969 Nixon “purge”, administration was handled by an international board of directors in the private sector…
2. Nearly all “biomorphic” aerospace designs were inspired by the Roswell spacecraft — from Kelly’s SR-71 Blackbird onward to today’s drones, UCAVs, and aerospace craft…
3. It was Ben Rich’s opinion that the public should not be told [about UFOs and extraterrestrials] . He believed they could not handle the truth — ever. Only in the last months of his decline did he begin to feel that the “international corporate board of directors” dealing with the “Subject” could represent a bigger problem to citizens’ personal freedoms under the United States Constitution than the presence of off-world visitors themselves.”