Well ... if it's there, it should be used. If it's not getting used, then you probably have a syntax error up higher in the file. Either that or the file you're looking at is on your PC, and not the one on the server (because the Developers Toolkit would have found it if it was on the server and you were searching All Files).
There's no point searching in other files. That definition is in the english.php file by default. It's not defined in other files.
It's defined in english.php and it's "used" by reference in the 3 other files you mentioned earlier.




