[rapjr9]

To rule out an electrostatic or electromagnetic interaction with the vacuum test chamber (some stainless steels are magnetic, some are not, but all are conductors that current can be induced in) you could put a strain gauge on the test chamber and suspend the test chamber from the ceiling. If the chamber moves in the opposite direction of the test article then it's an interaction with the chamber and not true propellentless thrust. If the chamber doesn't react then it's another piece of evidence that something unknown is going on.