If I understand correctly, Gondolin was not protected by any magical or godly means hiding it from Melkor and his forces.
Now, the Echoriad mountains were pretty far up North, and also not that far (by flight) from Tol Sirion.
Why is it, that none of Melkor's servants/minions/supporters were able to just fly around and notice Gondolin? Was Melkor particularly lacking when it came to creatures of the air? He at least had Dragons, and maybe bats (Sauron had a vampire bat form).