The settings panel now has a checkbox for highlighting any neighborhood that is not connected to any roadway. This usually happens when a neighborhood is new, and you just haven't noticed it on the map. By default, the checkbox is turned on, so if you like this option there is nothing that you need to adjust.

If a neighborhood is on the map but it doesn't have any roads connected to it then periodically it will display a conspicuous warning from your current camera position to the direction of the new neighborhood. This should help a lot in finding new neighborhoods that spawned without being noticed.