Just get one of those fancy metal attaché cases, attach a capacitive sensor to it with some sort of ground plane inside the case, and wire it to some logging mi

Just get one of those fancy metal attaché cases, attach a capacitive sensor to it with some sort of ground plane inside the case, and wire it to some logging microcontroller. Add a gyro chip to boot. Honestly the embedded gyro should be enough, the MEMS chips in phones are CRAZY sensitive, and would easily detect being shifted by a cm.