Thanks! Are you thinking of one location in particular? Most of the time, I prevented players from going back because it's the easiest way to signal everything there has been completed
I think it was the cold water area. I had completed the part where the juice or whatever it was was created, but I hadn't explored the rest of the area, and when I went to go back, I couldn't. I did another run through, and there was a lot more to discover in ohter areas of the city.
