Ive noticed it on the ships several times, and not all the times in the rooms. It took a while, and I think I know where its coming from, since we have the same problem where I work on occasion. On the floors of the bathrooms, are drains. Under the floor they are designed to hold a certain amount of water, to keep gasses etc. from backing up. If the traps, dry out for any number of reasons,(normal evaporation, wave action etc) the smell will start to come out into what ever room the trap is in. All you really need to do, is to take 2 or 3 glasses of water, and pour it down the drains in the floors. Not the 1 in the bathtub, the one on the floor. It should solve the problem. It kind of came to me after talking to a room steward who said they had solved the problem in 1 room by taping off the floor vent/drain, and here at work where in a brand new bldg, the water that is supposed to go in to the drain, everytime you turn on the sink, didnt work because debris got into the lines. Simple fix, pour a couple of glasses of water into the floor drain and see what happens.