Last night, I was woken up by my carbon monoxide alarm. 
It was three beeps after five seconds I think.

The reason I didn't call anyone was the circumstances didn't really add up.

I have two alarms, an old one and a newer model. The newer model is in the kitchen where my gas cooker and my boiler is and this wasn't the one going off. It works fine, I tested it.

The older one that has been sitting on my dresser in the living room was the one going off. The heating wasn't on, there's nothing that I can think of as to why it's going off.
I opened the window nearby and eventually, it went off.

This morning, I closed the window and placed the alarm back where it was, it hasn't gone off since.

Should I call someone or is this a false alarm?

Hi 

First step is to see how old your CO alarm is and whether it needs replacing. Different brands' CO alarms work between 5 and 10 years. There should be a 'replace by' sticker on the unit.

Second step is to make sure the batteries are still working correctly. If the CO alarm has replaceable batteries, these might need replacing.

Other factors that can set off a CO alarm include boat/car batteries being charged nearby or new floor screed.

Harry