It seems like this page would include that info.
Topic on Manual talk:$wgLogos
Ooh! I found this "(Note: The default value of this variable depends on other variables, such as the values set in Setup.php, after LocalSettings.php is executed)" but I would note that anything in parenthesis is, by definition, not going to grab the eye.
I suggest that there should be a line that something like "Variable is set in: LocalSettings.php" or something similarly obvious. I looked at that page several times before I noticed the line in parens. And of course that was right after I asked because you can't find the answer until you ask someone.