I'm currently working on a parser that reads complete LaTeX logs. Most of the log format is, though weird, easy to figure out, but these square brackets are puzzling me. Here's an example from near the end of one of my logs:

Overfull \hbox (10.88788pt too wide) in paragraph at lines 40--40
[]$[]$ 
 []

[102]) [103]
Kapitel 14.
(./Thermo-141-GrenzenFundamentalpostulat.tex [104

])
(./Thermo-142-Mastergleichung.tex [105]) (./Thermo-143-HTheorem.tex [106pdfTeX 
warning (ext4): destination with the same identifier (name{equation.14.3.3}) ha
s been already used, duplicate ignored

Can anybody give me a hint what these square brackets mean? I can't see any structure in them.

I have the suspicion that lines 2/3 above are some kind of ASCII art representing the box layout, though I know too less about badboxes to justify this or identify the meaning of the single characters.

Then, the "[104" etc. seem to correspond to the page numbers, but I am still not seeing the reason why there is sometimes something inbetween the square brackets (like the pdfTeX warning above), and sometimes not.