Dried-out trap
Water in the trap forms a seal that keeps sewer gases out. In a rarely used basin, shower or floor drain the water evaporates and odour gets into the room. Run the tap and see whether the smell goes away.
Deposits in the pipe
Fat, food scraps, hair and soap build up on the pipe walls. Organic deposits decompose and smell, and they also narrow the pipe so water drains more slowly.
First steps
Clean the trap and the drain grate. Do not pour pan fat into the sink. Dose a biological product with bacteria and enzymes according to the pack, ideally in the evening when the drain is not used for a while. Caustic cleaners and disinfectants weaken bacteria, so do not combine them with a biological product.
When to call a professional
If the smell persists with a full trap, water does not drain or the odour is noticeable throughout the house, the cause may be missing sewer ventilation, a leaking joint or a problem in the connection. A biological product will not fix that.