Locate without opening
Describe whether the smell is strongest at the toilet, cabinet or pipe route and whether there is visible moisture. Include recent work or long non-use.
Keep the unit closed
Do not remove covers or mix cleaning chemicals. The exact model and existing installation help plan assessment of the relevant connections and serviceable components.
A useful distinction
An odour with visible wastewater leakage needs that leak stated explicitly. A smell without visible water remains a separate enquiry, and a strong fragrance should not be used to hide evidence before inspection.