MIT researchers designed a self-powering, battery-free, energy-harvesting sensor. Using the framework they developed, they produced a temperature sensor that can harvest and store the energy from the magnetic field that exists in the open air around a wire.
You can’t harvest energy from a static magnetic field without putting more work into it than you get out. This harvests fluctuations in the magnetic field that are given off by high power electronics like motors. This isn’t free energy, it’s recollecting a small amount of wasted radiated energy.
There’s no such thing as free energy overall, but there is within a frame of reference. The sun acts as “free energy” to the reference frame of Earth but obviously comes from its fusion reaction radiating energy. Similarly, if you hold the sensor as the reference frame, it’s getting “free” energy from magnetic fields that wouldn’t be used otherwise. So I don’t think it’s wrong to describe that way! Passive energy collection is pretty neat.