If you have a sunroof, I would imagine water got into the sensor and you have corrosion forming in the connector.
If you want, you can grab the rearview mirror with both hands and rotate it counter-clockwise about 90 degrees. This release the mirror from the holder and you can disconnect the plug and see. Before you get to the plug, just pry the plastic side covers back, so you have better access.
If you have a sunroof and corrosion, ask the tech to inspect drain tubes and makes sure they are not blocked by some type of debris.
Yes, I know you have warranty but this post may help others.
Good luck!
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