My ground floor bathroom intermittently stinks of sewage. The toilet flushes fine and usually clears any waste with just one flush. What I find particularly baffling is the sewage smell happens even when no poo has been flushed away or even if the toilet hasn't recently been used at all. I thought maybe there might be a crack at the base of the toilet but I can't see anything and can't smell anything if I bend down and sniff around near the floor. Does anyone know what might be going on?