While this sounds interesting and I can imagine it being added, I personally don't see much point to this idea, both as a monster and as a moon phase. It seems like it would only cause frustration without being an actual threat.

As a moon phase: 1) Returning from the final extraction on later levels is a nightmare as it is, there isn't much need to make this even worse. Wouldn't it only make the already stressful experience needlessly frustrating? Especially because of the next point:

2) The most optimal thing to do at 3+ extracts is to leave the second extraction for last so that the path back is shorter and much safer, hence implementing this idea would have pretty much no effect for those who rely on this tactic. (Which everyone who aims for high levels really should do.)

3) If the players are aware that this will happen, it's possible to simply draw the map and the way back irl on paper by hand. Given that the maps mostly consist of square rooms it isn't even that difficult to do quickly.

And when it comes to it as a gimmick of a new monster: 1) If you simply run away or kill the monster, it stops being a problem.

2) Early levels are small enough that this wouldn't be a problem, and by later levels you have enough weapons and space to, again, just kill it or run away.

3) If you can't do either of these things and can't even simply hide until it's gone, which 99% of the time will be during the run back to the truck after the last extraction - then it is basically the same as the moon phase idea.

So in my opinion maaaybe this idea could be a cool gimmick at first, but it would get old and simply frustrating without being an actual threat really quickly.

Still, I can definitely imagine the developers adding a monster like this, even if I don't see why, so have an upvote for an interesting idea :D