Buying a CO2 meter for the office - especially a closed-door small room one - is something almost all indoor workers should invest in. As you noted, it gets over 1000ppm quite easily.

My office is an interior one without a window to the outdoors, so I have to flap the door open here or there along with the exterior door about 100 feet away. HVAC doesn't do much to help.

If I am in the office with the door closed for 2 hours (I share it with one other person), 1000ppm happens regularly.

Some good reading for any passersby on the topic:

https://www.jeffgeerling.com/tags/co2