[A_Brandybuck]

Besides the possibility, that Morgoth could have noticed the new upcoming union between the Elves of Beleriand, if Maedhros have hidden his strength and have let the Gap open, through the traffic of the messengers, there is another detail.
I am sure that Ulfang and his Easterlings had reported the plans to their secret Master. I cannot imagine, that the Easterlings in the service of Morgoth had not alarmed Angband.
They were greedy for the wealthy lands of Beleriand and would not miss out such a chance to proof their loyality to Morgoth.

Anyway the treachery of Ulfang and his people was the deciding factor of the battle and without it, Morgoth had not won the battle.