Trickling Flood attack
In the last part of the level, if you keep clear of the tower then the Flood attack can continue for quite a while. If you want to try this, I suggest going around to the left when you exit the structure where you met up with the Marines. That's what I did, and after coming to a bit of a clearing, a slow trickle of Flood went on for several minutes. I forget whether it eventually stopped or whether I just gave up waiting. I think I also heard a few lines of Marine dialog I hadn't heard before, which was a bonus.