Homepod Mini bursting into life on their own

Hi

I have three HomePod Minis, in three separate rooms of my house. They are not linked. Totally randomly Siri will burst into life e.g. this morning in a room with no-one in it and no noise, he suddenly started saying that he would get the information and send it to my iPhone; or he says he cannot find a song but will play music from my Library.

Spoke with Apple support, who could only suggest rebooting my network router, which I have done. Also dis a reset on one of the HomePod Minis the other day, but didnt cure the problem. I'm starting to get irritated with this.

Does anyone have any ideas ion what is happening and how too stop it? Many thanks