@Mamabearwhere this exact thing happened to me with my last pregnancy! I’d had 2 previous breath tests where I’d scored a 1 and a 2 and then randomly I had one where it came out as a 6! I did everything you did, got the boiler and my car checked, bought a C02 alarm and just generally panicked!
I went back the next week to retest and it came out as a 2, the midwife wasn’t concerned, in fact her first question was had I been painting, as apparently that can raise your levels, especially if you’re using a high voc paint. The air pollution where you live can affect it, I even read somewhere that if you’re anxious and have been shallow breathing, that can raise levels as well (something to do with oxygen saturation levels I think, I can’t remember exactly why now) The midwife also said she’d seen levels well into double figures so 6 isn’t massively high compared to what it would be if you were smoking.
I would push for a retest for your piece of mind and I’m sure you’ll have a normal level when you retest. I’d also be questioning them on why they were referring you to the stop smoking team if you’re a non-smoker, that seems a bit strange!
Over the course of my pregnancy I scored 1,2,3,4 and 6 so it was so varied and I dreaded taking them! I now have a scarily smart 18 month old who is bright as a button so I really wouldn’t worry about a one-off high score!