(no subject)

My carbon monoxide detector keeps going off in the living room.
This is, obviously, a worry. However, I bought the detector when I thought I might get round to getting the gas fire reconnected this year, and in fact there is no possible source of CO anywhere that I can think of; the boiler is electric, for example. This turns the expected potential single problem (OMG CO, will die, fix fire!) into two: is detector malfunctioning? If not, where hell is stuff coming from? Are the neighbours sneaking up by night and attaching their exhaust to the vent brick?
Obvious next steps: change battery (though it's definitely the alarm itself going off, not the low power warning), replace device. I expect that will actually sort it out, but if not, the next step after that isn't as obvious. Moving a detector round the house will tell me where there is and isn't a problem, but won't pinpoint a source. Get someone in, I suppose, but who, if it isn't gas-related?
(Step already taken: leave kitchen and staircase doors open, bring in fan.)