Team Brit Siri 
I recently had a weird issue in the middle of the night with Siri, so I feel your pain.
I briefly googled around and saw mention of this feature of iOS 14:
The Sound Recognition feature can detect a variety of noises, including a fire alarm, siren, smoke alarm, a cat or dog, an appliance, a car horn, a doorbell, a knock on the door, water running, and a baby crying. The idea is that even if you can’t hear the sound yourself, your device can. When it hears a specific sound, it will then display a visual notification to alert you.
I wonder if there’s a bug or other odd event that triggered Siri sensing that it heard a fire alarm?