I have been having this issue for a few months and fingers crossed may have have discovered the issue. I opened the boot and it started beeping. When I put my head inside the boot I noticed that the beeping was coming from behind the interior boot light on the nearside. Suspecting it maybe a faulty brake light bulb/ connection, I took out the bulb holder. On removing it I noticed that one of the female connections on the connector plug looked slightly corroded and the corresponding male plug too. I cleaned off the corrosion. The brake lights had been working fine previously with no warning lights on the dashboard. The boot light on the offside was blown but the nearside bulb still worked. As the noise was definitely emitting from the nearside boot light I fiddled with it but in doing so blew the bulb when reconnecting it . However the beeping has completely stopped. So check the light bulb holder for any corrosion or check to see if your boot light connections are faulty. 👍🏽👍🏽