In 1897, a UFO allegedly crashed near Aurora, Texas. The newspaper reported, "the pilot of this ship is supposed to be the only one on board, and while his remains are badly disfigured, enough of the original has been picked up to show that he was not an inhabitant of this world." In 2008, an unmarked grave was found in the area, but the grave was eroded and radar couldn't prove what type of remains it contained.
In 2014, the Rosetta probe finally reached Comet 67B after a ten-year journey. However, an anonymous source claiming to be a "NASA whistleblower" said that the "real" reason for the mission was that 67B was actually an alien spacecraft with external mechanical features, and that NASA had detected mysterious signals coming from the comet. That part is true, at least. You can hear the strange signals here.
"The Estimate of the Situation" was a document that's alleged to have been written in 1948 by a U.S. Air Force captain, stating that the extraterrestrial hypothesis was the best explanation for UFOs. The document subsequently vanished, but a similar top secret USAF memo still exists, and states that: "these objects originate from some previously unknown or unidentified technology, possibly outside the Earth." Blimey.