It's located in the lower right side of the map, and is unclickable. http://subeta.net/explore/centropolis.php

Maybe I'm going crazy, but has that always been there? If so, what's in it?

I think it has always been there but I'm not sure if highlighting it/labelling it was a mistake considering it seems not to link to anywhere...

could part of the plot / future plots take place there? it does seem a bit important.

Yeah, I know the building itself was always there, but the labeling seems new. Don't remember ever having a city hall.

[edit] After a quick google search, it has been labeled since the cruise plot, but never clickable. The highlighting and red outline just threw me off.