Is there a drain near the bottom of the stairs?. If there is lift the cover off to see if there is any water in it. The trap might be dry allowing sewer odors to enter into the house. might have a harder time smelling them right near the floor as it will collect up near the ceiling or in your case the top of the stairs.
Pour about 10 Litres of water down it if it looks dry or low this will fill up the trap (you can't see the trap) and block the odors. If it works problem solved just remember to check it in case you have a crack in the pipe causing the water to slowly leak out. Should be good for months but the water will eventually evaporate again so throw a few Litres down it every 3 months.
Hope this helps.
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