Magnetic anomaly or not, in bad weather conditions or at somewhat rough seas or water convections, any plane without a dipping sonar would have zero chance to locate any object in the water and even sonars could show zero.
But simulating too much accuracy in the game could give the player a big advantage and make the game boring.
ATM sinking subs or ships with 1 torpedo is ok for reality matters, but then the devs should make a better AI evasion. For example I never see subs evading in the vertical, they always stay horizontal. I find them too easy to hit, but here too one can say: it's just realistic... Difficult to feed every aspect.
But sorry, I digress, its' about planes. My first post just reflected a tutorial mission that was not even supposed to focus on planes and ships, it just happened that I 'tested' a plane. Doing things out of plan could confuse the game routine
In the campaign that I'm playing since I don't see 'magic' every so often, planes and helos behave ok in my opinion, so my first impression of 'magic' is only a single one, maybe I was going too fast near the surface, the water was too calm and so my shape was visible, I can't remember exactly but these are all possible reasons for spotting my sub all the time which would explain the constant overfly. And the plane just happened to have no weapon stocks loaded.