I spent a week there in April, Pro-Dive had just began operations. Nitrox was not being used yet. So, what gives? Are they still at the Allegro?
I think all the Nitrox on the island comes from one source, the big fill station that air supplies most Ops that don't have compressors. Odd that they didn't have any but then some Ops do require night before requests for Nitrox because it has to be retrieved from the fill station. As to your other questions...
How common are CO issues?
Even DAN admits that they don't really know, which is a real sore point for me. But then DAN only knows what is reported and CO deaths are easy to hide usually, reported as drownings; deaths are generally rare in recreational scuba so one might think that CO death in particular are only a part of that - which sounds good but not enough for me. Anyway, the word I got in emails with DP was that no one could be found on island with a CO tester to check the tanks.
How is a diver to know if shops are checking for CO?
You inspect their compressor to see if one of these is inline...
If anyone ever sees one on Coz I'd like pics please; I don't think there is one anywhere. They do cost about $1,000 USD to set up plus educated maintenance and I guess no one there thinks that the extra pennies/tank is needed.
I check every tank. My dive bud who makes half of my trips with me saw his first 5ppm reading in New Mexico last month. I don't think he supported my cynicism before then. He dived the tank content that he knew it was 5 ppm and not 50, but most of you divers don't know.
We did a Cenote Dive and stopped in the middle of a jungle "where the air is clean" for our tanks, quite impressive and made sense... Air they compress has no city smog.
On the road to Dos Ojos? I know that station, pretty impressive - but I did smell smoke from charcoal production that day. I did not know enough to look for that CO inline monitor back then, but I did check my tanks.
Oh the 5 ppm we saw last month was in a town of 2,000, no smog, electric compressor, best situation - but it still happens. Worse is possible.
So if you don't test your tanks yourself, all you have is
"deaths are generally rare in recreational scuba so one might think that CO death in particular are only a part of that," as even with the inline monitor I test. But if those odds are good for you, cool; I am such a cynic that I had seatbelts before hey were required and wear an inflatable vest on moving diving boats - even tho I am not a safe person by nature.