This is so very true. I registered specially to support this argument. A discreet notificiation would be very valuable, especially when having my meter around me, but not wanting to keep an eye on it the whole time.
I came here today to request the same - I carry my monitor with me a lot, but it's very obvious to pull out and check. I'd love for my Apple watch to discreetly alert me when co2 gets to a certain level.