On my flight back to YYZ from YVR today, someone apparantely caught smoking in the washroom. This is the first time that I have ever seen this in my whole travel career. The first time "she" set off the alarm in the washroom, the FA made an announcement "This is directed to whoever was smoking in the rear lavatory, please do not do it again as this is a very serious matter". Apparantely, she did it again, the FA took her up to the fron and talk to the Captain. When we arrive at the gate, two cops was waiting for her.