Possible Clues:

• "Close Encounters of the Third Kind" sightings
• ETs' rides
• Carriers for little green men
• Subjects of some supposed government cover-ups
• Some oddities of uncertain recollection
• Space travellers start unusual fiction output scientifically
• Fleet from outer space?
• Space saucers, briefly
• Wormhole ships
• Strange objects visible to all, loud and huge