As such there is no possible way that dementors could ever relay information to someone not already in possession of the knowledge.

Guess that idea is wrong, then.

Auror Li and Auror McCusker had rearranged their chairs around the table, and so they both saw it at the same time, the naked, skeletally thin horror rising up to hover outside the window, the headache already hitting them from seeing it.

They both heard the voice, like a long-dead corpse had spoken words and those words themselves had aged and died.

The Dementor's speech hurt their ears as it said, "Bellatrix Black is out of her cell."

There was a split second of horrified silence, and then Li tore out of his chair, heading for the communicator to call in reinforcements from the Ministry, even as McCusker grabbed his mirror and started frantically trying to raise the three Aurors who'd gone on patrol.