Well technically, any modifications you make to GPL code are still owned by your employer. Now if the modified code gets distributed by your employer, THEY are

Well technically, any modifications you make to GPL code are still owned by your employer. Now if the modified code gets distributed by your employer, THEY are the ones that are under obligation to make the source available. But if they don't redistribute any of the code, then the GPL specifically allows them to keep the changes proprietary.