What are the smells emanating from my spare-toilet room, and are they dangerous?

My toilet is flushed regularly; but this small room (5foot square) seems to fill, on and off but with regularity, AND at different hours,with the odour of what seems gas, which seems to smell exactly like BOTTLED kitchen gas for the cooking stove. The odour goes after an hour or two as mysteriously as it came, even if i leave the room's door and window shut. It does NOT come out of the toilet-BOWL; NOR does it come through the window's "air-leaks" (this window opens on to the drainage/sewage shaft of the block of flats in which i live.). But if i open the window during the smelly episodes, NO smell comes in from there.This smell has been bothering me for some 3 years, now; but NEVER happened in the first years. So if it does not come from the shaft where all the sewage-discharge-plumbing is, and also NOT from the toilet-bowl itself which always contains the normal level of water in it, where the deuce does it come from; WHAT is it, and is it poisonous/dangerous? It seems different from the smell of ammonia ociated with long-standing urine in UNflushed loos. What harm can it do to humans?