Do you have any seldom used sinks or other drains, such as a floor drain in the house? They all have a trap, or at least they should, but if they aren't repeatedly provided with water from normal use, the water will evaporate and leave the "P" trap dry which will result in sewer gases coming up through the line. It could be that the trap under your bath has come apart and the water is leaking out. This is unlikely if the bath is on a floor where one would notice the leaking beneath, but a leak into a seldom used crawl space or basement might go unoticed.
A final suggestion... all drains have to be vented. The vent line is (at least here in the U.S.) a PVC or cast iron pipe in older houses that goes all the way to the roof of your house. These can become blocked for various reasons which will cause the sewer gases to back up through the overflow drains in sinks and/or bathtubs. Blockage is usually at the top of the pipe stack, such as a birds nest or somesuch. Best of luck!