That certainly suggests some sort of ‘alarm’, either set up in the Sonos software, or some third party system, such as Spotify Connect or Apple HomeKit, or the like. Have you submitted a system diagnostic within 10 minutes of experiencing this problem, and called Sonos Support to discuss it?

They would likely be able to see where the command to play came from. 

I'm sorry, but that's just funny.   I wish I had a solution for you.  I don't think that I'll be buying a robot servant any time soon. 

I've never set an alarm and it started about 2 months ago. 

I'll try the diagnostic route when I'm next in when it happens. I've never set an alarm, was unaware that you could. 

Problem is that its a really loud alarm which could be bothering an elderly neighbour when I'm not here to turn it off. How can I disable it when it doesn't show that there is an alarm?

The problem is multi-faceted, unfortunately. The ‘alarm’ doesn’t have to be in Sonos’ software, it could easily be in other home management software that uses the Sonos for output. It could also be a Spotify Connect issue. This is why engaging with Sonos Support is paramount, you need to track down what is triggering the alarm. 

The easiest, of course, is to check the alarm settings in Sonos. While you may not have set it up, it’s always possible that someone else did it, either accidentally, or as a prank. 

The only way I can think of to stop it from going off without doing any of the above is to unplug the speaker every night.