Grnite is SLIGHTLY conductive so if your counter top goes anywhere near live wires such as an appliance, you may sense it is hot with thaat detector. The stray field from the microwave is jumping onto the granite... HOWEVER please verify that your ground on the microwave is intact and returns to the third wire and has continuity to the case of your breaker box. If the ground is broken, that will certainly cause the indication. You can get one of those receptacle testers to test this.
ALSO NEVER trust the detector when safety is an issue. The device is "indication only" and it can be wrong for various reasons.
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