David Wisniewski's book The Secret Knowledge of Grown-Ups" mentions in passing a hangar wherein employees of the United States Government "stack all the banged-up flying saucers", implying that the existence of such is well-known, but that the Government attempts to conceal it.
The Thorgal series by Grzegosz Rosinski and Van Hamme (29 albums) where the main protagonist Thorgal Aegirsson is in fact the son of ancient astronauts.