The meter was reacting to how you held the phone, not to what was in the wall.

A phone's magnetometer reads the magnets inside the phone itself as well as the field around it, and that internal offset can be ten times larger than Earth's field. The app used to cancel it by taking a single reading at startup and subtracting that number. But the phone's own magnetism stays put while Earth's field swings through the phone as you tilt it — so tilting alone moved the reading by as much as the whole scale. Sweeping a phone along a wall is nothing but tilting, which is how bare drywall could show a red "live wire" warning.

The app now works out the phone's own magnetism properly, by fitting the shape your readings trace as you turn the device. Turn it slowly in a figure-8 for a few seconds when you start and it will say when it is ready. After that, rotating the phone in an empty room barely moves the number at all, where before it could swing across the entire scale.

Also in this update:
• Warnings like "live wire" now wait until the calibration is actually good. Until then you see the reading and a neutral "still settling" label instead of a hazard verdict.
• If the phone's magnetism changes mid-session — a magnetic car mount, setting it on a speaker — the app notices and recalibrates instead of reporting the change as a field forever.
• The guide no longer says the app measures AC magnetic fields. It samples about ten times a second, so 50/60 Hz mains fields, RF, Wi-Fi and radiation are outside what the hardware can do. What it does measure is distortion from ferrous metal, magnets, motors and transformers.
• The ICNIRP figures are labelled as reference levels for context, not a compliance verdict. They are the 50/60 Hz public values, and this app measures a static field, which that guidance treats separately.