The usual cause of summer washroom odour is a dry P-trap: the water seal in a seldom-used floor drain or WC trap evaporates in the heat, letting sewer gas back into the room. Flushing the traps with water weekly restores the seal.
What a P-trap does
Every drain has a U-shaped bend — the P-trap — that holds a small amount of water. That water is a seal: it physically blocks sewer gas from rising back through the drain into the room.
Why it dries out in summer
In Qatar's heat, water in a seldom-used floor drain or guest-WC trap evaporates quickly. Once the seal is gone, there is nothing stopping drain odour from entering — which is why the problem appears in the rooms you use least, during the hottest months.
The fix (and when to call a plumber)
Pour a litre or two of water down each floor drain and run each seldom-used WC and basin once a week through the summer. That alone resolves the vast majority of cases. If the smell persists after consistent flushing, the issue is usually a blocked or inadequate vent pipe — at that point, call a plumber.
For managed assets, Makan schedules summer trap-flushing as standard so owners never get the call in the first place.