Maybe it's "ships in system" + "PDCs on planet".
I'm not at home so I can't check my current game, but I'm pretty sure in one system where I have a PDC on the main planet but not on a secondary colony, the main planet no longer has unrest messages but the smaller colony does. I just go and stick 1-2 garrison battalions everywhere and largely ignore the unrest messages. Since civilian mining colonies auto-generate a garrison battalion, they are a great source of troops as you simply relocate them when the civ mines exhaust the asteroid, just before you abandon it.