Didn't we have this discussion already? I prefer to mark such
functions as \internal. That makes it obvious that it's internal, not
that you just forgot to doxygenize your comments. And some users like
the doxygen indexing and interface to browse their internal functions
too - they have the option this way to just ask doxygen to include
\internal functions. They can't do that if such functions have
non-doxygen comments.
Sure, then if you change the comments to doxygen ones, mark them as
internal :-)