ODST: Side mission repeats?

I was in ODST clearing the streets and a beacon came on my screen. When I got to it, it was the helmet stuck in the wall. And now I am playing as “Buck” again. I already did this mission once. Since the beacon came on, is this telling me that I missed something in that mission or do all the beacons stay on so you can go back and play those areas again later? If not, how do I get out of the “Buck” mission and go back to being the “Rookie”? Any help would be greatly appreciated, Thank you.

You can quit the level and go back to the streets. It’s just there for replay value.

i dont understand u

How do I quit the level and get back to being the “Rookie”?