Spanish intelligence from a captured fishing-boat revealed the English fleets had joined.

Evidence

That night after midnight one of the Spanish pinnaces, commanded by an English-speaking officer, came back with a Falmouth fishing-smack it had picked up and her crew of four. From them the duke learned that Howard and Drake had joined forces and been seen that afternoon, standing out to sea. — Garrett Mattingly, The Defeat of the Spanish Armada, 618

Categorization

• corroborated: 2 independent passages support this claim — Garrett Mattingly, The Defeat of the Spanish Armada, 618