Case insensitive without redirect?

I have several WordPress sites that I’ve inherited managing. On this one particular site, you can use any case on pages after the URL. For example, http://www.example.com/TeSt will load the page http://www.example.com/test AND it will not redirect/change the case in the browser. The page loads and the URL in the browser remains http://www.example.com/TeSt. You can do this for any URL, /test, /media, /whatever. They do use your plugin. Is this a function of your plugin? They want the same functionality on their other sites so I’m trying to track down how this is being done since it does not appear to be on the web server level.