What model TV do you have? Are you using an external media device?

Some TV’s allow audio to pass through to a soundbar from a cable source when in Standby. It’s especially so when an optical connection is used. The solution is to turn the cable box off as well. 

So I have an LG oled tv

no external box the arc is just connected to the tv through the hdmi cable 

just use freesat

Doesn’t Freesat use a set top box?

No it built into the tv

According to this thread, it seems to be an issue with LG TVs:

Does it continue to play audio if you use any other app on your TV other than Freesat?

No so if I’m using Netflix, now tv, Disney plus or even the ps5 this does not happen 

only if I’m watching the freesat then turn the tv off

The Arc will play whatever the TV sends it. If theTV sends the Freesat sound while the TV is turned off, then the TV is passing that data through the ARC cable to the Arc. Which means it is neither an issue with Freesat, or Sonos, since neither receive knowledge that the TV is off.

I would recommend first checking for a software update for the TV. If that doesn’t resolve the issue, I’d get in touch with the TV manufacturer’s customer service, and ask them why the TV is sending a signal out while it is turned off.