Anger, Sadness And Envy Episode 1: Arrival and Sierra 117
Let me point out, where you mentioned how people say units stay at the spawn points...
I highly HIGHLY doubt they played on Legendary or even Heroic. Normal is EASY, it's that simple, it's built for people who are new to Halo, which is sad since most reviews are built of Normal.
In simply heroic, on say Crow's Nest where there is that large group of brutes and one with the hammer. If you simply pick them off over a certain amount of time, the brutes will form groups of 2-3 and track you down in your hiding spot. IE, they WILL track you down, and if your not ready you'll die because of that.