NetworkError 404 // undefined in FireBug

I get the following error pop up in FireBug on almost every page. Thought it was something wacky in jQuery but I've commented out my scripts and it still pops up. Sometimes on a delay. Any ideas??:

NetworkError: 404 Not Found - [mysite]/index.php/[controller]/undefined

I am on CI 3.1.0 / PHP 5.6.10