I haven’t tested it but I believe AirPlay can be used without being on the same WiFi network using peer-to-peer networking; sort of how AirDrop works.
The following talks about it but I have not tested to see if that is still the case with current versions of the various OSes.
Thank you for your thoughts. Sadly, I checked and the Home settings are restricted to allow only this home, which I assume is only the people I designated, i.e. my wife and I. it’s pretty locked down here–I don’t belive it could have come from outside the house, however much that makes the most sense. I guess maybe I’ll unplug them unless they’re in use, I dunno. It’s just so weird. I didn’t even know HomePod COULD announce a fire alarm.
Do you have any smart sensors connected to HomeKit?
Only motion detectors to run the lights in the night should we need them, and then to turn off a minute later.
My thoughts that the alarm was caused by a smart fire sensor that made the HomePod announces the alarm. That doesn’t sound to be the case.
What’s normal? HomePods can play music, news, podcasts, etc. A network breach is possible but I wouldn’t discount anything based on content.
Yes it does. But my HomePods have exhibited similar behavior so I have a different opinion.
Your suggestion of changing passwords is a good one. I have a Due reminder to change mine every few months and use 1Password to create them. You can’t be too careful these days.
That’d be a minor miracle, given I have no such device. 
Another weird thing about it is that I don’t use the default siri voice (use British male–makes me feel like Iron Man), and that’s the voice it used. I’d have thought a prank wouldn’t leverage my personal settings.
Seems like a network password rev is in order. I’ll start there and see if it solves the issue. Thank you to all for the thoughts!
BTW, noticed there is an update to the HomePod out.
Version 15.1.1
Release notes state it fixes an issue where Podcasts could fail to play.
is it possible that someone set a single instance alarm on your homepod, it will speak the alarm title if you give it one. My kids have done this a few times, very annoying.
Seems most logical, but I can’t imagine how it could have happened. The only person other than myself with HomePod access in the house is my wife, and even assuming she’d set such an alarm (which she never does) she absolutely wouldn’t have set anything with “fire alarm” as a title. It’s really good suggestion, but I can’t see how it’d be possible in our house.
FWIW, I’ve been away for a bit and plugged everyone back in with a new password in play and have had no issues so far. I suppose it was just some kind of fluke.
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?
I’m that case this is spooky, good idea to change passwords.
Also perhaps do a sweep of devices that are associated with your iCloud account just to be sure.
I’m sure you’ve also got two factor / two step with enabled.
Good luck.
Actually Team Canada Siri here. 
Yes, I seem to recall you had some repulsive music happen at an ungodly hour without a by-your-leave. Your suggestion makes the most sense of any so far, although being the middle of the night, I wonder what the HomePod thought it was hearing to warrant a fire alarm. If that’s a bug, it’s a good one. Box hears some rumour it believes and then trumpets it to the house. It’s like living with a conspiracy theorist. 
So far, the incident has not recurred, so I remain cautiously optimistic that it’ll be ok going forward. Thank you for your thoughts!
This EXACT thing happened to us last night, except it was about 3:30. And the “alarm” stopped when we went downstairs to see what it was doing.
It played an alert song (not one I’ve heard before but very similar to its timer sound) interspersed with a very calm “fire alarm” in the voice we use. So, so weird and I can’t find anything else about it anywhere.
Well, for what it’s worth, I’m grateful to not be alone in this particular weirdness. I never did get to the bottom of it, but it’s never recurred, so I am going to assume it was some kind of odd spoof.
I am very pleased you posted about it and that I’m not going mad! It definitely happened and it’s exactly as you described. We also did the sleepy fire search… I’m going to test a few things and if I find anything out… I will let you know haha! So strange. I was so confused.
I’ve only gone and bloody worked it out. I found a timer set deep within the home app. I made it do it again, it made the same alarm sound and it announces the name of the alarm which, in this case was “timer”… “timer alarm”. It says “timer alarm” with a different emphasis each time and a couple of them sound JUST like “fire alarm” because it barely says the “t”. I think it must have happened on one of the many occasions we’ve said “hey Siri, set a timer for X minutes” and we’ve thought it’s not done it. But it has in fact set an alarm called “timer” for 04:00am, not 4 minutes. Do you use yours for timers? I really think this is it haha.
Just a thought - have you ever set a timer for a “fire” for any reason? Or a reminder to turn off a gas fire or something haha?
This has not happened to me, but there are so many possible reasons:
HomePods misunderstanding what is going on (it does not take much for a HomePod to get triggered when watching TV: it just needs to listen when you are watching TV. If Siri “thinks” that it has heard “hey, Siri” there is not much more needed to “understand” that a timer has to be set up quoting stuff from the TV show or whatever else. Like I said, it never went that far over here, but Siri has reacted to stuff that was running on TV on several occasions at my home over the years. It might be a good idea to tell Siri to just delete all timers followed by delete all alarms after something like this has happened.
pranksters inside of your home telling Siri what to do
pranksters with access to your WiFi (this is where the WiFi security/passwords come into place)
a mishap while somebody was trying to use AirPlay nearby (or again: a prankster using AirPlay) - there are a lot of options for AirPlay - Play audio on HomePod using an iOS or iPadOS device - Apple Support
This makes the most sense of anything I’ve ever read around this. We don’t use the default Siri voice, so I totally buy that our voice could be mistaken for “fire alarm” in the dead of night when I’m still mostly asleep. It also makes complete sense that Siri could have gotten the wires crossed when I set a 3 minute timer for my tea and then went off at 3am.
The only thing that still doesn’t compute for me is that the typical behaviour for timers on the Homepod at our place is to simply play the usual alarm noise. It’s never announced the name of the timer, nor any words of any kind. However I’m willing to accept that somehow, whatever I said (or Siri heard) when I presumably set the timer that woke me in the night somehow set it up to do just that.
Thank you so much for figuring this one out!