What you should avoid doing is leaving live cables embedded in walls
that are not within the expected cable zones (i.e. inline with a visible
accessory, or within 150mm of a corner or wall ceiling junction). Adding
labels / notes is obviously handy to help future maintainers when doing
something non obvious.


My house seems to be riddled with diagonal runs, installed, by the
look of things, when it was built 40-ish years ago. I guess the fact
that they are all covered with galvanised capping makes them ~sort of~
alright, although that was part of the reason why I removed the
redundant cable that led to this question (that, and the fact that it
provided an outlet in a small cloakroom - just seemed easier to be
done with it). It makes guessing the route of a cable a lot more
fun......