Hmmm...this situation sounds familiar. In the past month I've had 4 calls where the CO detectors were sounding but our really expensive, recently calibrated CO meter detected nothing. When the homeowner purchased a new detector the problem went away. All of the suspect CO alarms were battery operated units that were 2-4 years old.
I have had good experiences with the Nighthawk detectors but, any unit that is AC powered with a battery back up and a digital display is a good choice. While there are occasional problems with individual units, most major manufacturers build a good reliable product. It is important to note that like smoke detectors, CO detectors also have a life span...usually 7-10 years. Be sure to read the instructions for the specifics of your detector.
I realize that members from all over the world will read this and I can not speak for every fire department. However, in the northeast U.S. most fire departments have CO meters and we are more than happy to come and check your home for CO. Personally, I do not look for "violations" when investigating a CO problem although I will point out hazards to homeowners to help keep them safe. I don't have any interest in "writing up" anyone. I believe that I'm there to help the residents not to whiz in their Wheaties.
My best CO story:
We got called to a raised ranch style house where their 3 CO detectors had activated earlier in the day but reset themselves. I asked the resident where the furnace was located and he told me "we have electric heat."
"Oh" I replied, "well where is your water heater?"
"It's electric too" he replied.
"Do you have any gas or oil fired appliances?" I inquired.
"Nope" replied the homeowner.
"Do you ever warm up the car in the garage?" I asked hopefully.
"Never, we always pull out the car and shut the garage door" he replied.
I silently swore with a puzzled look on my face. After a powow with my crew we realized the only possible source of CO was the car. We had the homeowner pull the car out and shut the garage door. In short order the CO alarms were sounding! It turns out the wind was blowing the CO past the nonexistent garage door seals, into the garage, and finally into the house. I don't think I've ever worked that hard to solve a CO problem before or since.