Called to a customer's, Dr.s. Said called alarm co. called fire dept.because of alert. Fire Co. and gas co. came. I mistakenly went to Dr.s office because they both work there during the day. Wrong, they said the wife home and was taken to the hospital. Went to house, no on there, garage open, car there but no one answered and door was open. Purse and receipt from hospital on counter. Freaked me a little when she didn't answer but she did when phoned. Big town home.
Anyway, the daughter said they checked everything and focused on a water heater on 3rd floor. But tried and tried but couldn't find anything but gas guy red tagged it anyway. Well the son in law listened to this and then chimed in, "what about the carpet guys"? They had backed up to the garage, ran hoses in, had door open to garage and ran the truck powered carpet cleaner the whole time, 2.5 hours with the wind in the right, (wrong) direction. Closed the doors and left. 15 min before alarm.
Still left the water heater tagged. BTW, it's on the other side of house from her bedroom where she was. Dropping off an NCI monitor, ( didn't want one before when offered.....)
I would definitly put a CO monitor in the house but I'll bet the carpet guys are the culperites.
With the Water heater off, check for 100% closed gas valve. Make sure there is no flame on the burner. Start Dryer, and all exhaust fans. Strike a match near water heater, blow match out and check at draft hood where the smoke goes. Start water heater and repeat draft test with match. BTW about three years ago a carpet cleaning company backed up to the double door at one of our remote sites and sent people to the hospital. It occurred after they left. There aren't any fossil fuel appliances there, and also no fresh air. It was capped off during a remodel. The really stupid thing is we aren't allowed to carry our own CO or CO2 detectors.