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.

You are aware they have deployed passive and active sonobouys from aircraft since WW2, right? Also only active sonobouys show on the map.

Yes, and I imagine this is what happened: the plane has spotted me the first time with the help of a sonobuoy (active or passive), then he got to my position and dropped another active sonobuoy; I went away but was again spotted, and rinse-repeat for the plane dropping more sunobuoys etc.
I'm now convinced there's no 'magic' but just a chain of events that got me spotted again and again, it must be hard to evade these sonobuoys !

Planes have MAD sensors (they detect the magnetic disturbance the mass of you boat creates.) That can certainly appear to be "magic." These same planes can also get intel from other detector assets in the game through "networking" and since they fly faster than helos, they cover more water, faster. Once another asset reports your position, the plane can be there in moments.
-Pv-

Thread name should be changed into:
"Do the planes use some 'MADgic'?"