The Zanuka Hunter has a 1.5% of spawning for marked players on Corpus missions within the first four minutes. Each additional marked player in the squad increases the chances by 0.5%. Upon spawning, one marked player will have their mark removed, much like the Grustrag Three. Running additional invasion missions for the Grineer is also a good way to find the Zanuka Hunter as the chance is doubled after getting the mail for that invasion.
Only the sentence about the additional chance is wrong. Marks for Zanuka and Gustrag Trio dont stack (different from Stalker, where they don´t enlargen the chance to meet him). But in invasions, you meet more people which have a mark, because the probably do invasions more often and thereby have a Mark. Don´t Zanuka spawns only in Corpus and Gustrag only in Grineer Missions! (Invasions count as missions of the faction you fight against.) You can buy the beacon which calls it from Baro if you want to farm Zanuka (much less work/ time intensive since you can get the needed Ducates much faster). Hope I could help :-)