Hi everyone, I hope you don't mind my first post being a request for help, but I've been trawling the internet for days and read about every website/forum post there is about gas smells in buildings.

I live in a 140 year-old(ish) granite building that at some point has been converted into four flats. Because it was all one big house before, we do get noise and cooking smells etc travelling between floors.

Anyway, about 4 weeks ago now we noticed a gas-like smell in the communal ground floor hallway, and we also seemed to get 'clouds' of it moving through our (first floor) kitchen and hallway, above where the ground floor smell was. It dispersed quickly when we opened the communal hallway window, though.

A week later the smell was coming more frequently (3 out of 7 days) and our neighbour called the Emergency Gas line. We had a couple of SGN guys come out and test everything. Unfortunately the smell had mostly dispersed by the time they arrived and they detected nothing after a thorough examination.

A week later we called them out again, and more or less the same thing happened - smell had mostly gone by the time they came, and again they tested all along the gas pipes, everyone's boilers, cookers, etc - everything they could think of. They found nothing, and said it couldn't be gas, it's just something else with a similar smell.

Another two weeks on and we're still getting the smell, although not every day. It's almost always there when we come home in the evening, and seems to disperse as we open a window (or when everyone else in the building comes home and fresh air gets in). No-one is usually in the building during the day, and our boilers are not usually on either (yet it appears anyway). Once the smell disperses, it doesn't reappear during the night or early morning (also a time when no fresh air gets into the building).

I imagine that it's a small amount of some sort of gas (not necessarily natural gas since it tested negative) getting in and building up during the day, as it does go away quickly when fresh air circulates. I am concerned about the week over Christmas because no-one in the building will be at home. If whatever it is is left to build up over a week...

Of course we could leave a communal hallway window open over that week when no-one is around, but we are worried about pipes freezing. Also, I just hate not knowing what it is! The emergency gas guys told us not to worry, it will probably go away by itself in time, and it couldn't be a gas pipe leak as that would be constant. But I've read that sewer gas and so on can also be combustible, so I wonder if anyone has any ideas what this could be, so we can get it sorted before everyone goes away?

Thanks very much, and sorry about the long post. I just wanted to make sure I gave as much detail as possible

ETA: We have a carbon monoxide detector in the area of our flat where the smell is, and it's happy enough and staying quiet.