Intrusion into the left margin is not registered as a bad box as far as I can tell in left-to-right typesetting because, as Werner pointed out it isn't really possible to intrude into the left margin other than intentionally in this case. When you intrude deliberately by setting a negative space, it is assumed that you know what you want and therefore the intrusion is not bad.
For example, consider the following MWE
\documentclass{article}
\usepackage{showframe}
\begin{document}
\hspace*{-25mm}ABC \hrule
\end{document}
This certainly protrudes into the left margin

but LaTeX does not consider it a bad box:
grep ful <filename>.log
returns nothing.
It is the same principle as you see if you put something which is too large for the space into a box and tell TeX that the box has a width of 0pt. TeX will not complain that the contents is too big for the space or not really 0pt. It will assume that is how big you want it to think the box is. This trick can be used to put an oversized image on a page without triggering a bad box warning. But it means that you must manually check that the content fits (e.g. does not spill off the paper) because you will not get a warning about the width of the box in this case regardless. After all, you told TeX explicitly to assume it fitted in zero width. Unlike Word, if you tell TeX that P or to do Q it will assume P or do Q. It will not assume that you really wanted to do S or that you actually meant that R.