How does discoverability work regarding chaos cults and skaven undercitys?

Ive been playing a new run of Karl Franz, and the past 2 times there has been a strong source of Tzeench corruption in my province after clearing out the seccecionists. I am running some mods like mixus faction remixer and immortal empires expanded.

Any tips for finding the source of it outside of building buildings that raise the discoveravility of the cults?

UPDATE: As it turns out, just a basic night watch building and 2-3 heroes was enough to root out the cults. I wish yhere was a more active way of finding them, but this worked well enough. Thank you everyone for the advice