Mileage,trip and outside temp readings stay on

[Bullnose75]

Mileage,trip and outside temperature readings on Tourer dash stay on all the time now even when no key in ignition and car is locked. Any ideas what causes this guys ?
This started happening two days ago.
Day before this started happening the outside temp reading was reading -2c even though outside temp was 10c but after a short trip for shopping this started reading correct outside temp again.

Road tax due April 1st,might sorn Rover for a month as wont be using it while in lock down. Should I disconnect battery leads in case dash readings staying on drain car battery ?
Paul