On the same day this thread was started, our ensuite began exhibiting a foul smell also, although not one we would have associated with the waste pipe. Can't describe the odour here.
We use all the sanitary ware in the ensuite every day, so ruled out the running dry thing. Checked the attic for odour, other rooms and could not find the source. Settled on letting time solve it, but with a plan that after a week we would do more investigation if it had not cleared.
Today it is gone. So either time did heal, or something has resulted in the root cause getting shifted.
So don't go ripping up floor boards without at least giving time a chance to heal the problem.