I have been processing a number of files using the same environment  
file. Each time, early in the log file I get the very same message:
...
(/usr/local/teTeX/share/texmf.local/tex/context/base/pdfr-ec.tex)
Overfull \hbox (13.2pt too wide) detected at line 63
[]
system          : module ancientgreek loaded
(/Users/acbowen/Library/texmf/tex/context/third/greek/t-ancientgreek.tex
loading         : module ancientgreek
...

The problem is that I can see no evidence in the document of an  
overfull \hbox. Moreover, if I change the font from SIL-Gentium to  
Latin Modern, the message goes away, though it returns once more if I  
use GFSDidot say (only the line number differs).

Can anyone explain what is happening or suggest how I may proceed to  
determine what is happening and why?

Alan