[inlikealamb]

>However, if a couple of these civilizations start to openly seek contact to others, what can the hidden ones do?

Stay hidden or destroy anyone close enough to put you at risk along with themselves. This is a spoiler, but IIRC from the books the ultimate safety net was to make your solar system not only invisible... but impenetrable in either direction but essentially trapping yourself in a black hole... thus removing yourself from the equation and hopefully satiating anyone watching.

The time scales and distances involved meant that you weren't really perceived as a threat until you approached the ability to reach light speed, which made you stick out enough to be noticed in far corners of the universe. We broadcast radio, but it's not loud or far-reaching enough to be noticed by the far-out civilization destroying overlords. It was loud enough for a different nearby civilization to come destroy us in an attempt to save themselves from being destroyed along with us.

Of course, when applied in reality who the hell knows.