Carbon monoxide alarm went off

Yesterday in my apartment my carbon monoxide alarm went off for a brief period. I was away for the weekend and when I arrived home all was normal. Within 5 minutes of me arriving back, the alarm started sounding.

I reset the alarm, opened the windows, and that was it. It didn't sound again, even after closing the windows. The device seems to be fine - indicator is blinking as it should be and the test button works.

The only burning appliance I have is a pretty much brand new gas boiler which was installed in February. While it would have been on in the morning for a while, the alarm started to sound at least 4 hours after it would have switched off.

Is it likely the boiler is the source? Since it's an apartment block the only other theory I have is that it came from someone else's apartment. There are small vents in my ceilings which seem to link to a common shaft, as sometimes I can smell food being cooked from other apartments, so it could have come through them?