A lot of the alarms beep when the battery is almost flat, and they do it at full volume, usually in the middle of the night. Very annoying. It's just a short beep though, usually every minute or half minute. What happened to you doesn't sound like that.
If you press the test button on the two alarms, does it sound similar to what you heard? Can you tell which alarm it is that way?
Obviously if it's the smoke alarm, you can easily see that nothing is burning, so that's fine. If it's the carbon monoxide alarm it's a bit more difficult, but if your heating and your cooker are all off, there doesn't seem to be anywhere it might come from. My carbon monoxide alarm has a display with a graph that shows what it has detected over the last few hours, and it's usually zero of course. You could always get one like that, if you don't have one already, then you'd get some reassurance that the carbon monoxide level isn't going up and down for some reason.
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