They're not going to be nice and give us a heads up that they going to scourge the planet. If they decide that we committed too much heresy, then they're going to make sure that we can't run before a surprise exterminatus fleet shows up right at our doorstep.
What do you mean by heads up?
Like an exterminatus weapon isnt something you carry around in a briefcase. We would see the ship coming, and have opportunity to shoot at it. Like we are going to notice a warship flying into the system. You dont really just randomly roll up on a planet and wipe it off the face of the galaxy. Some rando would be bringing a warship to our planet, and we would be very curious about who they are and what they are doing.
They would also need to actually investigate to determine that exterminatus-worthy heresy is taking place. Different inquisitors are equipped to study different things. If a maleus inquisitor shows up, they arent going to be equipped to identifying xenos heresy, or techno heresy.
We are a colony at the ass-end of nowhere. We dont get ships just casually rolling up on our doorstep to drop people off allowing an inquisitor to just slide on in. People come here specifically to visit us, and we would probably want to know why, and unless we are a threat to the Imperium, which we would never become, why waste an exterminatus weapon on us?
Those are for stuff thats an actual threat to the Imperium. We are not and never will become said threat. You drop that shit on Chaos incursions or Tyranid invasions.
Not tech priests with a disdain for orthodoxy. If you really took objection to that you could call Mars and they would probably want to send a fact finding mission, and they would likely just try to take over before resorting to exterminatus. What, do you think that young inexperienced Tech Priests didnt get too curious about Eldar guns before? Or that a Colony never took the opportunity of being in the ass end of space to say "Fuck the rules!"?
There is a whole *list* of reasons why fearing an exterminatus just for bucking the Mars orthodox line is a waste of our time.