Weird unasked-for file trace in LaTeX

[Doug McKenna]

I can't tell if this is a bug or by design, but ...

Execute this MWE for the LaTeX format (TeX Live 2017):

  \tracingcommands=3
  \tracingmacros=1
  \tracingonline=1

  \documentclass{article}
  \begin{document}
  Hello, world!
  \end{document}

This sends a whole lot of tracing to the terminal, in which the following lines appear towards the end of it all, when producing the final single page:

  \@arabic #1->\number #1
  #1<-\c at page 
  {\number}
  {the character 1}
  {\hfil}
  {end-group character }}
  {internal vertical mode: \relax}
  {end-group character }}
  {end-group character }}
  [1{/usr/local/texlive/2017/texmf-var/fonts/map/pdftex/updmap/pdftex.map}]
  {\endgroup}

Some sort of a complete file read occurs smack dab in the middle of printing out "[1]".  But notice that \tracingfiles has not been set.

Is this a bug?  Shouldn't that parenthesized file path not be output unless \tracingfiles has been set to 1?

Doug McKenna
Mathemaesthetics, Inc.