[qwerty123]

Just watched on replay and can't help but notice the lights are on in Studio E with someone sitting at the desk? Seems weird if that studio is supposedly out of use today.

Given it’s not meant to be in use today isn’t it ‘not in use’ rather than ‘out of use’ which would suggest it couldn’t be used for some reason or other.
Studios that are not in use for live broadcasts could be being used for rehearsals either for technical reasons (to train new staff or to test some altered settings) or for major events (for example the kings death), being used for external filming (eg a film cameo), to prerecord something for future use ( for example a lot of the review 2023 stuff will probably be being recorded at the minute). It could also simply be a case of visitors to the BBC taking photos in there for social media or other uses.