Historical clue formulations

• Signs in the 2002 sci-fi film "Signs" — 2020-01-03
• The signs in the movie "Signs" — 2013-12-03
• Mysterious phenomena … or a clue to solving 17-, 24-, 33- and 45-Across — 2006-06-29
• Alien art form, some say — 1996-12-24