I keep a wooden skewer in there, to check water levels in the shower and hand basin U bends. Always OK. Have tried keeping the plug in the basin, and putting tape over the basin overflow and shower drain. No difference. No marks on the ceiling below, but I lifted the floorboards looking for a tiny weep or similar, but nothing.
The toilet discharges horizontally, into a 90 degree bend that fits inside a pipe coming up through the floor. I can see some sort of rubber seal or doughnut perhaps half an inch down the pipe from the floor. Just by chance realised that the pipe from the toilet is not a tight fit in the rubber seal - I could move it about 1/8th of an inch backwards and forwards. Filled the gap with sealant and smoothed it with a wet finger and job done! That was a couple of weeks ago, and no smells since then. Result.