Many non-smokers have been suffering in silence, not knowing the source of the secondhand smoke that drifted into their homes, so I thought I would share our experience here. This was how we correctly identified our 3 smoker neighbours:

1. Next door smoker: we spoke to all the residents on our floor and confirmed that none smoke. But after the next door neighbour moved in, we started getting strong secondhand smoke that filled the entire lift lobby on our floor. We also had to close our toilet window permanently to prevent his smoke from wafting in. Plus his upstairs neighbour complained of his smoke too. After he's made aware of the impact of his secondhand smoke on others, he tried smoking at the ground floor for a while, but is now back to his old ways again, though smoking in his bedroom toilet now, that doesn't affect us as much as it affects his upstairs neighbour. 

2. Upstairs smoker: we got very intact cigarette ashes on our balcony, with some landing on our bed sheet. Since we did a round of mail drop (notice) at our stack to appeal to smokers above us to stop flicking ashes at the balcony, and the neighbour immediately above our upstairs unit told us she's not a smoker, based on how intact the ashes were, we identified our immediate upstairs neighbour as the smoker responsible and spoke to him about it. Things improved.

This smoker tenant was subsequently replaced by another smoker tenant, who was hostile to our appeal to smoke downstairs, insisting that smoking in his own home was not against the law. So we had no choice but to apply for mediation via CMC. Attached is what the CMC invitation letter looks like (sorry it got truncated). He moved out after his lease ended and is now replaced by a non-smoker tenant, as the property agent got to know about the problem of smoker tenants on our wellbeing and now tries to do better tenant screening to avoid smoke issues.

3. Downstairs smoker: Based on upstairs smoker quitting smoking due to his wife's pregnancy (found out from MA after our feedback) and occupants in unit below our immediate downstairs unit told us they didn't smoke (opened door for us and we detected no smoke), we identified him as the one whose smoke affected us. He refused to answer the door when we knocked on several occasions, so we applied for mediation via CMC. Fortunately, the smoker is a tenant and has since moved out after his lease ended.

My advice is to talk to your neighbours on the same floor and +/- 2 floors to "exchange notes". Try to talk face-to-face with your smoker neighbour(s) first. If the situation doesn't improve, then consider mediation. 

CMC invitation letters to attend mediation are sent FOC to both you and your smoker neighbour. You the applicant pay $5 if both agree to proceed with mediation. I am part of the official statistics of 71 applications from Jan 2019 to Jul 2020 cited by Min. Shanmugam at last month's parliamentary sitting. CMC application numbers are tracked monthly, so more cases will attract the attention and further action from the authorities.

Hope this helps.