I gather that you recognize this already, and others have alluded to the same, but if you're concerned about either the 1ppm CO itself or what it says about your compressor setup, it would probably be best to first independently confirm the reading. The Analox has a stated range of 1-50ppm with good accuracy, but as a general rule for analytical instrumentation, reading at the lower limit of detection shouldn't be taken accurate for granted, without regular and specific verification of the unit in question. Dandy Don seems to know these well, maybe he has knowledge of what Analox or users have done to back a claim of a 1ppm quantitative limit.
I may not be clear on the question, but I'll explain my approach...
1: When my Analox portable CO analyzer was a year old, before my next trip, I sent it in to be checked. They don't think it's necessary to calibrate since it has the field calibration, but as new as much of this is - I wanted it tested. It passed without any needs.
2: For the field calibration, I prefer to be on the water and not moving, as urban areas can have elevated ambient CO and a moving boat can have a stationwagon-effect of exhaust - but in the real world, I'll do the best I can.
3: I'd ignore a 1 ppm as negligible and possible just rounding or my jiggling of the unit. 2 or 3 ppm gets my attention, and I have dived 5 to 7 ppm - but won't now. What if my field-cal was in 5 ppm ambient air and the unit is giving me 5 to 7, which is really above my field-cal setting?
4: I am also calibrating my Pocket CO unit before my next trip and taking it & bags for backup testing.
5: And of course, I'd invite the Operator to test while I watch if he has a unit. I would be surprised it he did, and still wonder if it was calibrated, but we could compare. If he hasn't got one, screw it - mine wins.
The good news is that with more and more divers field testing, the Ops seem to be trying to protect their business and us by supplying cleaner air.