NDIR or not - cheap CO2 monitors

On piece of paper, I'd sharpied a square 5 mins before shooting this clip. The CO2 monitor I bought from Amazon was marked as NDIR type, and was shipped from China many months back. I'd taken it out of its case some time ago and it is not so mobile these days.

In the video, the monitor is showing an appropriate CO2 level for a home office with windows & doors closed and with me in the room for a couple of hours now.

I place the square of sharpied paper to each of the two sensors in turn.  Watch the CO2 readout shoot up  when CO2 levels in the room has not changed at all.

Hypothesis: this would happen for a TVOC * class of CO2 monitor but not for an NDIR ** one which is regarded as a superior technology.

* TVOC stands for "total volatile organic compound" and the US EPA suggests that checks for "any compound of carbon that participates in atmospheric photochemical reactions". This can be crudely correlated with CO2 levels.

** NDIR stands for "nondispersive infrared". This should not be changed via this sharpie test (yes a genuine Sharpie).

Conclusion: The monitor I have is TVOC type not NDIR as advertised on Amazon. The NDIR class of monitor would not do this for the same sharpie test.

Note: Removing the piece of paper (after the video) sees the level slowly drop down to the expected high 500's low 600's level.