What would cause a catfood smell when you dont have cat food?

My apartment always has this yucky cat-food smell. I dont have any cat food. Ive worked to keep it clean but the smell just continues to linger.. I can open up the windows but it comes back when I get home at the end of day and the place has been closed up. I cant leave the windows open during the day because they present an easy target for thieves! What would cause this kind of odor?