This is the thing with Cloak, it seems they kept watering down his powers over the years to the point where he's just basically a guy in a cape that can trap people with his cloak.
Originally, his cloak was a doorway into another dimension, the Dark Dimension. This was the same place that NC tps too, Darkstar/Blackout/Vanisher/etc.. get their power from. There was never a mention of a Beast or demon living inside, that came later (and is crappy writing IMHO since other Darkforce wielders make NO mention of such a creature).
Cloak was always intangible unless he strained himself and made himself corporeal. Cloak isn't intangible in the same way Kitty Pride is intangible. She goes intangible (Vision too I think) by spacing her molecules apart so they pass through solid objects. Sufficiently dense objects have been known to stop her from phasing through (I think a Sentinel pulled this trick on her or something in the future). Cloak is intangible because he does something with his physical body that shunts it into the Dark Dimension. It's an apples and oranges comparison.
So Magneto sensing NC tping into and out of the Dark Dimension by feeling the ripples in the magnetic lines as Nightcrawler enters and leaves the Dark Dimension would be meaningless vs Cloak because Cloak's body NEVER leaves the Dark Dimension unless he strains to become physical. He wouldn't leave any ripples in the magnetic lines because his body is intangible. His intangibility is nothing like Kitty Pride's/Shadowcat's since he's not spacing his body's molecules apart to allow him to pass through solid objects, his entire physical form is shunted into another plane of reality.
Now add to the fact that he can create pitch black inky smoke that can't be penetrated by normal means and Magneto would be FUGGED in a fight vs Cloak. His only hope would be to maintain distance and flee.
Now these are his "classic" powers. It seems he was raped by the nerf bat in recent years. Shame too since he was one of the few "cool" and powerful minority characters in comics back in the day at Marvel.