I reread the OP and this made me scratch my head.
Low and mid priced electronic scales will suffer drift due to a number of factors but if a beam scale starts moving after it has settled there has to be a outside force acting on it such as a air current. Energy is needed to set a mass in motion.
Newtons first law - An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force."
Air currents, vibrations or a poltergeist, take your pick
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