In Half-Blood Prince when Harry sees Dumbledore’s memories, he sees Voldemort ask for a job at Hogwarts.
Dumbledore refuses, saying he knows all about the Death Eaters. Voldemort is furious, telling Dumbledore he is as “omniscient as ever.”
To which Dumbledore replies: “No, no. Merely friendly with the local barmen.”
In The Deathly Hallows, of course, we find out who that local barman is.