OK - every day’s a school day and having just stuck my head into the books over a coffee for a refresh, I got it wrong-ish.
BPG4 (page 16) says that “Presence of circuits that cannot be readily identified or traced” warrants an FI code. And both BPG and 7671 state that an FI = an unsatisfactory result. So that sadly is that as far as the OP here.
Where I come back to my ‘ish’ is that it’s the job of an inspector to determine whether or not something is safe for continued use. We have all seen test sheets (mostly on here) with circuit after circuit full of LIM - making them largely meaningless but passable. Yet if you can’t find the end of one circuit in an otherwise 100% spanking install, simply because the heating engineer (a theoretically skilled and competent trade) has had to hide it somewhere in an un-obvious location then you have to return a failure and an instruction to the client that they’ll have to dismantle the place brick by brick until you’ve found it??!