Should my carbon monoxide detector be reporting zero PPM?
No, 400 ppm is NOT a safe level.  Here is the US Govt. CPSC information on CO:
CPSC CO Information
To quote the relevant part:

What CO level is dangerous to my health?
The health effects of CO
depend on the CO concentration and length of exposure, as well as each
individual's health condition. CO concentration is measured in parts
per million (ppm). Most people will not experience any symptoms from
prolonged exposure to CO levels of approximately 1 to 70 ppm but some
heart patients might experience an increase in chest pain. As CO
levels increase and remain above 70 ppm, symptoms become more
noticeable and can include headache, fatigue and nausea. At sustained
CO concentrations above 150 to 200 ppm, disorientation,
unconsciousness, and death are possible.