The Jargon File (version 4.4.7, 29 Dec 2003):

hidden flag
 n.

    [scientific computation] An extra option added to a routine without
    changing the calling sequence. For example, instead of adding an explicit
    input variable to instruct a routine to give extra diagnostic output, the
    programmer might just add a test for some otherwise meaningless feature of
    the existing inputs, such as a negative mass. The use of hidden flags can
    make a program very hard to debug and understand, but is all too common
    wherever programs are hacked on in a hurry.


The Free On-line Dictionary of Computing (30 December 2018):

hidden flag

   (scientific computation) An extra option added to a routine
   without changing the calling sequence.  For example, instead
   of adding an explicit input variable to instruct a routine to
   give extra diagnostic output, the programmer might just add a
   test for some otherwise meaningless feature of the existing
   inputs, such as a negative mass.  The use of hidden flags can
   make a program very hard to debug and understand, but is all
   too common wherever programs are hacked in a hurry.

   [Jargon File]

   (1994-11-24)