This could be output a ton of different places and ways, but judging on what you
re saying, and that you can’t see anything in your header.php, check your theme functions.php
.
Look for something along the lines of this:
dirname(__FILE__);
pathinfo(__FILE__, PATHINFO_FILENAME);
SCRIPT_FILENAME
If no luck still, check index.php
and then check mu-plugins folder for files.