Where in BS7671 does it state the absence of additional rcd protection MUST be a Code 3 observation (see page 427) and I emphasise the word MUST, no references to third party guide books.
It doesn't. On page 427 There is guidance that says it must be given a minimum C3 (item 2, guidance for the inspector). There is also mention of the definition of all the codes including FI (items 7,8,and 9 guidance for recipient).
The definition of FI excludes it from use where an item can be fully inspected and tested within the scope and limits of the report. FI in this instance is not therefore suitable for use.
A C2 may be applied to such a situation depending on cirumstance, but not a C1 as a lack of RCD could not be construed as "danger present"