Detection questions

[Slim]

I have several detection related questions and hopefully my mates can get me squared away.

1: Is there much risk in leaving my radar turned on due to enemy detecting my radar?
If so, it seems as if my radar auto turns on when I surface, is this right?

2: There are circles around enemy ships on the nav map that I assume represent detection areas. For destroyers there is a small area in front which I assume represents and uncovered active sonar area, then an outer area, which I assume is an active sonar area, then another outer area which I assume is a visual detection area? Can someone confirm this?
2b: Also do these circle areas stay the same at night and are representing a daytime condition (range), therefore are useless at night? It seems that way to me.

3: Are the radar switches on the bottom control panel out of sync with the radar station? It seems when I switch it on at the bottom panel and then go to the station the radar screen is blank and it is not showing sweeping unless I turn it on at this radar station?