Detector of ferrous metal, and maybe magnetized watches. – LEPSI - Watch MAGNETISM Review

Firstly, the app works nicely for what it is. But it should be known that the compass chip in the iPhone will, like any compass, be affected by ANY ferrous metal. Whether that ferrous metal object is magnetized or not. So if the app tells you that your mechanical watch is "magnetized", you are more than likely getting attraction to a ferrous part (the strap springbar or clasp are likely culprits). If no strap, then the mainspring or small screws & parts may be enough to pull the compass, even if not magnetized. If you're demagnetizing over & over and still getting a "magnetized" reading, this is why. Perhaps watch the strength level, if it drops, you may have removed some magnetic contamination, but you'll always get "some" reading from clean steel, if present. All similar apps like this work this way, so not a problem, so long as you are aware.
Review by JSH:RxM on LEPSI - Watch MAGNETISM.