Found it. Someone tackled it on some thread here that I never saw apparently. What it boils down to is that the 15% unrest is sort of a base amount for that given province in a lot of cases (it could be from 3 points worth of spies too, though - they cause 5% per point of spy skill). The file involved is descr_rebel_factions.txt, which lays out what rebels different regions can have (actually it sets up the units involved in each rebel event, and doesn't explicitly name the regions it applies to). Apparently all regions that can spawn rebels do so at 3% chance, and each percentage point corresponds to 5% permanent unrest in that region, making the 15% we so often see. Cities that can have 0% unrest therefore cannot spawn rebels at all.

The original post that details the find is here (including the text of the rebel file):

https://forums.totalwar.org/vb/showp...0&postcount=19