TC reported smoke external alarm at RearM1 car. TC changed ends checked and confirmed no signs of h eat or smoke . No other abnormalities spotted. ~~~~~ TC reported smoke external alarm at Rear     Check Tims FC-735/721,Further check found smoke detector water seepage into PC and cause the fault.Replace smoke detector and function test ok. Smoke detector- Old s/n: NIL    New s/n: SD-017 Done by Koh. 24/08/21