I'm using a modified Telu GUI elements mod. Looking at the relevant files (*borders.png), I can tell it comes from those. But also in other mods the same line is there, both in Cabido's mod and Telu IV.
What exactly is the thinking behind this? Is there something special/buggy with this map, as it contains a lot of rocky areas? I just think it looks strange.
Example using Cabido's mod, as you can see, it's not related to occupied territory, just around rocky hexes:
