Walk into - or should I say I had to go into a house that had a gas powered 3" trash pump running in a basement stairwell pumping water out of the basement - had to move suction lines around. The CO levels would have to have topped any level you could possibly get even at 100' - CO poisoning provides me a very distinct feet and you are very much aware of the problem long before it becomes a recovery, very much aware. You will feel the pounding and tingling and you have to know you can't keep standing there.
I've worked on stock cars, drag cars and owned a landscape company - I've had all sorts of engines running in confined areas and experienced it enough to know that "feeling" - it is very obvious.
Most if not all CO related deaths I have ever heard of have come from poorly plumbed furnace or water heater or someone running a generator in a garage - the deaths were from people sleeping while the incident occured - not wide awake, senses full on.
I would test tanks filled remotely by a gas compressor, tanks filled by some small one off operation or such - but you very rarely hear of problems, fill stations have monitors and IMO, you'd be well aware something was wrong long before.....