My current idea is that they're the replacement team for the last Inquisitorial team that was sent to Xomula. Also, everyone on the planet already knows they're there for exactly that. When they first come off the landing thunderhawk, they'll be greeted by one of the ruling Magos and a full honor guard from the Plague Militia.
The real secret is that the PCs are really just the 'face' team the Inquisition tries to keep on the planet at all time, to keep people happy. The face team's entire job is basically to prove that the Inquisition is there, keeping people safe. In all truth, all they really do is the occasional odd job from the locals. The real Inquisitoral teams are deep under hiding, and from every Ordos imaginable. The only reason the planet hasn't been Exterminatused is because it's to valuable a resource to destroy. Also, I'll try and start them off slow. No real big Xeno or demonic incursions till they get ready for it.