Carbon monoxide and natural gas@ Radiator vents

Gasman

Had a service call where the CO detector went off,  Boiler system.
3O ppm CO in atmosphere where I arrived. I ventilated to 1st floor to 0 ppm,
went to 2nd floor , only one radiator, but CO in atmosphere was 80ppm, i ventilated to 0.

checked boiler when on= 18ppm, water heater 5ppm, gas dryer=0ppm, gas oven after warm up=80 ppm. All appliances CO were in range. 

Found out customer was bleeding air out radiators and let water run for 5 minute out of them,I put my detector over the radiator vents and got 4000 ppm(highest my detector could go) and 73 percent Gas
After this long story, what would cause these readings at the air vents of the radiators?
I have guesses but want other opinions. 
Never came across this before.

mattmia2

Is what is in the piing specifically natural gas or it is any combustible gas like hydrogen from electrolysis of the water? If it is natural gas then it is an evacuate the building until you figure it out situation.

ch4man

your CGI many be calibrated for natural gas but still will detect any combustibles.
a couple years back I was helping a guy on a boiler system. he was wondering if we were purging air out or if the fitting was sucking air in by using his lighter.
well it was purging "air" but it was in fact hydrogen and lit off.
its true one shouldn't be smoking while bleeding radiators

captainco

What kind of analyzer are you using? Is it an analyzer or a personal safety device?

Gasman

It is a professional analyzer.
It detects combination natural gas and carbon monoxide continuously.

The Hydrogen comment and able to detect any combustibles sounds on point.
I'm going to call the manufacturer to get there take.
Thank you all for your suggestions.
 
Well let you know what the manufacturer said.