I have this: https://www.airthings.com/view-plus
It ain't cheap.
Well price is no guarantee. Try the hand sanitiser test I detailed above.
The CO2 sensor in the View Plus id advertised as follows:
NDIR Sensor (Non-Dispersive Infra-Red): Measurement range 400–5000 ppm Optimum Accuracy ±50ppm ±3% within 10 – 35°C / 50 - 95°F and 0 – 80%RH after initial calibration time
Can you tell from that description whether it's a true CO2 sensor?
That's proper CO2 sensor then. The cheap ones are estimating CO2 levels from VOC levels.
Second this. It's possible that the 3000 reading is triggered by something else. They should try rubbing sanitiser on their hands and then holding the device - if it goes crazy then it's not directly sensing CO2.
I sort of did this test last night. I assume that VOCs include ethanol, so I mixed up some drinks near the monitor. VOCs went from ~60ppb to a peak of 579ppb. CO2 remained constant at about 640ppm.
I am less surprised than other people that making an open fire indoors increases CO2 levels. Where do the combustion byproducts go if not into the air.
A lot of cheaper CO2 sensors really detect VOCs and try to estimate CO2 from that.