First thing is put your nose right down into that tub drain. It'll be pretty strong so there would no mistaking where it's coming from. If it's not the tub then try the sink drain and under the sink and around the toilet. If the smell gets stronger with the fan on, then turn it on and you may be able to feel for air flow over the drains. If you can smell and feel the air coming out of a drain then something is causing your trap to siphon, could be lack of vent or lack of trap for that matter, lack of proper vent or "S" trap, studor vent, trap larger than trap arm, tailpiece too long, plugged drain etc. Did you recently remodel or buy a house that has been remodeled? If it's an older home you could have a broken pipe under a slab, or a pipe that has just been left open or abandoned in the wall. There's dozens of reasons but a good start would be to determine the age of the home and if any renovations took place since the home was built and in what fixture or approximate area the smell is coming from.