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I have a TIM10 that was once reporting negative CO2 levels one spring with the windows open. I'm not sure what I think about it. It's not a scam product by any means, it can definitely tell when someone is in the room or a window is opened or closed. I needed the space and unplugged it awhile ago and just don't worry about CO2 levels currently.

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My understanding is that some co2 sensors assume the minimum value seen over the last 30 days is 400ppm as a calibration. This could possibly explain a negative value if the sensor has been in higher co2 environment for some time, and is suddenly exposed to the outside.

You're right. The instructions say to place the sensor outside for a time to calibrate it, which I never did.

Does it require callibration?

Yes. All consumer CO2 detectors I’m aware of take periodic calibration.

Not Aranet4

Yes you do. Once a year: https://aranet.com/faq/

Their data sheet also advises calibration if used at “high elevation.”

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