I couldn't find anything; I think these skills are completely embedded in the engine.
That's a shame... I guess I'll never know for sure. The "Witch Gathering" skill has a 10% chance + 2% per hero level, including the first level (12% at level 1, 14% at level 2), or, in rare cases, starting from the second level (10% at level 1, 12% at level 2).
I also had a somewhat crazy (and possibly even downright silly) idea: the "Witch Gathering" skill isn't listed in DefaultStats, but the "Poisoner" skill of Lethos is, and they have similar text descriptions (10% chance + 2% per hero level). What if the "Witch Gathering" skill only "visually" has its own "unique" description and images, but in the game's code, it simply references the "Poisoner" skill for functionality?
That is, both have 10% + 2% per level, and they just use the same strings
But I couldn't find any confirmation for my crazy idea 
UPD: Just in case someone does find these strings/files not in the engine, but under very strange names, in addition to the "Witch Gathering" skill of Erin and the "Protector" skill of Ingvar, I also want to add the "Embalmer" skill of Kaspar (I also couldn't find anything). Maybe I'll add a couple more unique skills later
I just don't know why.
UPD: I figured out the "Protector" and "Embalmer" skills; I didn't find any strings, I just tested them in the game. But the "Witch Gathering" skill remains an unconfirmed mystery. Let's assume it's "including the first level": D