I have a CO problem, and can't locate the source. I have 2 gas log fireplaces, my cooktop is gas (although it's electronic ignition, so there's no pilot) and my water heater is gas (I should say that all of these are propane).

I've lived here for over six years with 3 CO monitors that have only registered 0 until now. Twice in the last 3 weeks they have gone up into the danger level, causing one of my cats to get carbon monoxide poisoning. The two times this has happened, I've had people out to check, but they can't find anything. Last night as a precaution I completely turned off the propane into the house. So right now the only thing with a flame is the water heater which the people lit when they came back today to check. When they came this morning, everything had dropped back to close to 0 because I had opened every window and door in the house last night. They left no stone unturned on the water heater, and checked other things also. They couldn't get any readings. When I got home from work tonight all the monitors are rising again. They suggested the possibility of something causing false readings, but I know it definitely has to be CO because of my cat's illness both times. The other puzzling thing is that it hasn't been an ongoing problem. It happened once two weeks ago, then things just naturally went back to normal, then is happening again now. There's nothing peculiar that has happened these two times. Does anyone have a clue as to what my problem could be? I should mention that my HVAC system is geothermal, so that can't be the culprit.