I recently solved a small problem for a restaurant I was visiting. I entered their restroom only to be assaulted with the smell of the sewer. The reason for the smell was simple. The water in the trap for the floor drain had evaporated and was allowing the sewer gasses in. Just like the trap under your kitchen or bath sink the floor drain has a trap too. If the floor is not cleaned by flooding it regularly, then it should at least have a cup or two of water poured down it to maintain the trap integrity. Once a week should do it.