When I went to UT Austin, they’d show films in Hogg Auditorium (maybe they still do). This was a big, old building dating from the 1930s, with a glowing greenish clock to one side of the screen, and creaky wooden seats. When the movie was playing, I guess it attracted moths and/or other insects, because often there would be bats flying around above the audience.
The first time I noticed this was during The Adventures of Baron Munchausen. We weren’t quite sure, at first, that we really were seeing bats: earlier that evening, we had made certain arrangements to attain a mental state more suitable for viewing such a film.
But it seemed like whenever I noticed it after that, it was always during a vampire movie: Nosferatu the Vampyre, Fearless Vampire Killers etc.