There’s a running gag in South Park that the town has been invaded by aliens. Don’t believe me? Check out this gallery of freeze frames that points out exactly where the alien has been spotted over the years. Supposedly he’s in every episode, and though I can’t verify that, he certainly showed in in these 30+, so he very may well be there.
Check out the gallery for yourself below, and if you’ve spotted the alien in a place not listed here, send me a screenshot with proof!