Sewage smell on middle floor

[HangTime]

OK, first of all apologies for the slightly crude descriptions herein, but rather than beat about the bush I'll explain the situation as-is.

I live in a 3-story townhouse and for a while now I have observed that if I go for a smelly 'number two' in the bathroom (2nd floor) that there is a lingering sewage smell (my personal fragrance rather than a generic sewage smell) on the first floor landing. I haven't noticed this smell after using toilets on the first floor or ground floor, so it seems to be something to do with the bathroom toilet or downstream flow from there.

I can't see any obvious signs of leakage, but all the pipes are hidden behind walls/ceiling/floor etc. The smell also dissipates after a while; I haven't timed it but say after a few hours it seems to have gone. I had some crazy vision of a hidden leak building up an ever-increasing pile of human waste in a wall cavity , but I would have thought in that case the smell would stick around indefinitely.

Any ideas on the potential cause or way of testing it? I am reluctant to call out a plumber until I can put my finger on a genuine problem, and don't really want to start smashing holes in interior walls/ceiling in the hope of locating the precise source of the smell.

 

[HangTime]

Thanks for the advice. Hadn't considered the possibility of it being caused by other routes drying and that that seems highly likely as there is an ensuite guest room on the same floor where the toilet, basin and shower go ages without being used. I should probably use it more but I prefer not to use the guest toilet for obvious reasons.

Also the toilet in the bathroom doesn't always get flushed that well... there's something not quite right with the button meaning that sometimes I have to lift the lid off the cistern to get it to fully come back up to normal position (if I don't do this, then it stays very slightly depressed causing a very slow trickle of water, which then in turn slowly drains the cistern causing it to annoyingly refill a bit every few mins).

I'll do some testing over the next few days and find out.