Most works, especially the more populous ones, feature many recurring characters who interact with the protagonists under different circumstances, and who thus wouldn't be expected to know each other. Or, if they do know each other, you wouldn't expect them to have a history together apart from what you see.
Sometimes, though, these characters turn out to have been Connected All Along: they actually have met or crossed paths at some point, or share some other sort of unexpected connection. There are times where it would make sense, they might run in the same circles, even if the topic never came up around the protagonists.
It's likely the characters themselves won't remember the encounter, making The Reveal as much of a surprise to them as it is to the audience.
This is a Super-Trope to One Degree of Separation, Long-Lost Relative, You All Share My Story, Already Met Everyone, Luke, I Am Your Father, Luke, You Are My Father, Your Son All Along, and Not So Episodic.
Sister trope to Chekhov's Gunman, which most of the examples here will also be examples of, and Forgotten First Meeting. Compare Ambiguously Related, which may precede this trope.
A secret connection between two characters is usually a surprise, so Spoilers Ahoy! There will be unmarked spoilers on here.
