the only thing I would consider hidden information is the HTML class attribute from a log message that decides which type of message it is.
For example, in chat the line:
SGT John Wayne stammers, "H-He...he j-just....."
is written to the log like this:
<span class='game say'><span class='name'>SGT John Wayne</span> stammers, <span class='message'><span class='body'>"H-He...he j-just....."</span></span></span><br>
As a user, you don't really see the "game say", "name", "message" or "body" other than which format is used for the text (it is defined by the class). I would say this is "inferred" information and any user could export the log and open it in a text editor to see this code.
Other than that, there's nothing that isn't also directly visible in the BYOND chat window.