I have the same perpetual 15% Unrest problem with Segestica (East of Patavium). I moved in a high-leve spy and he didn't find any nme spies in there. I also put up watchtowers ALL OVER that territory to make sure there wasn't some nme army or whatever messing up my grass, but nada there too.

It seems really odd that a lack of trade goods would make people unhappy. I mean, if it's a constant thing, don't live there. Unrest shouldn't be caused by things beyond our control.

Anyway, I still have no idea what's caused the Unrest. What would be a really kick-ass feature would be if, when you mouse over an Unrest block in your City Info Page, the pop-up text not only told you what it was (Unrest) but also what is causing it.