Sometimes sitting in a car in traffic can really raise the readings taken I’m not sure if maybe this could have been the case?
As keeley said, breathing in carbon monoxide from traffic can cause it to be higher. I have my intakes changed to internal only and don't open my car windows.
I had this at my first appointment. Totally panicked, bought a new carbon monoxide detector off Amazon etc etc. went back in the next day and asked for it to be rechecked. Was totally normal then (reading of 2). ? Faulty machine.