Huge Elongated skulls, aliens. Out of place artifacts, aliens. TPTB, aliens. Ritual mutilation/sacrifice, aliens. Mysterious human origin, aliens. Complex ancient structures, aliens. Religious/spiritual entities, aliens. Atmospheric phenomena, aliens
the list goes on, feel free to add.
because were modern day slaves, you want your slaves dumb so you teach them only enough to do there job. a slaves worth is directly tied to his obedience trust and faith in his "masters".