The battle is already underway on arrival. The hobgoblins are taking their stand, letting the smaller goblins in the back try their best at pop shots from the back.

Spoiler: rolls
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Gob 1 is firing into the crowd of guards, no real roll needed there.
Gob 4, however, seems to stand as a point man, and would likely be the first to notice anyone's approach (Passive Perception 13).