I'm pretty sure it's just a voltmeter/ammeter on the milivolt/amp level. Brain activity is just electrochemical impulses. When I turn my stick meter on for troubleshooting hvac equipment it starts out on milivolt levels and picks up what is called artifact, without even touching the leads to anything, until I manually range it up. What your experiencing just tells you that the sensor is working it is just picking up a stray current that may not be associated with brain activity but is the same level impulse.
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