The sensor is hanging on its electrical wires and hanging in free air with 30 cm clearance in x,y,z directions. I soldered the wires directly to the hall sensor. The magnet itself I bring by hand to the hall sensor. It's a simple bar magnet (no screws, nuts). One interesting thing I observed is that the hall sensor itself is attracted to the magnet when it is really close. So maybe something get magnetized in the sensor itself and therefore gives non-stable quiescent output voltage?