Thanks for the welcome.
It is an interesting subject.
When a home CCTV system looks onto public property like a path or road, or looks onto another's property like a neighbours house/garden, the guidelines are that the CCTV owner should use privacy filters built in to most good CCTV systems. With this you can 'draw' a black area over neighbours windows, or garden for example, so that it never records that area.
The other part is that when CCTV looks onto areas used by other people then data protection laws come into play, so the people being recorded have certain rights. If you were captured on camera say in the street or on your property by someone else's CCTV, you're protected by GDPR.
In that situation, yes the CCTV owner would need to put up a sign to state CCTV recording in operation, and also if people are being captured on camera, they can request a copy of the images/footage and the CCTV owner must comply within 30 days.
This is my understanding of it, but I'd like to mention that I'm not legally trained, so if you need proper advice on the subject in order to take some further action, I would of course recommend speaking with an appropriate legal authority.
Have a good day!