Brigand Lair
Devil's Den
Forest of Pain
The Demon Gate
The Prison of Hearts
The Vale of Infinite Horror
Those are all blood sites that cause unrest from 5 to 15 I believe.  At some point I got this data from one of Edi's databases which I used in the SemiRandom province and god editors.  I just did a quick scan and came up with those six.
How positive are you that another player has not just parked a spy (or ten spies 

 ) on that province causing unrest?