Just to follow the subject, I've got something the same in the kitchen. Sometimes. Does smell like drains to me. The kitchen drain goes into the main drainage via a u bend outside which has a drain above it for the patio. So smells can't really come up via that. I have an outside loo in the corner where the smell is most obvious - when it happens - but check there is water in the u bend. It's not much used apart from in the summer. Next door had an outside loo which backs onto mine - but had the toilet removed and their boiler fitted in there. The builders who did all this were related to The Lone Ranger...
I wondered just how well they capped things up after removing the loo - but next door say they haven't got a problem.