TC reported Smoke External Alarm at Front DT car. Rear T338 proceeded in ATPM mode feedback no signsof heat nor smoke on track. TC and Staff (ALJ) checked the train and confirmed no signs of heat nor smoke in train. ~~~~~ #3531 SMOKE EXTER L DETECTED.CHECK CONFIRM THERE IS NO FAULT.FURTHER CHECK FOUND SMOKE DETECTOR UNIT WITH SYMPTOMS OF WATER INGRESS.FOR  PREVENTIVE MEASURE C/O #3531 SMOKE DETECTOR UNIT.PLASEL APPLIED DONE.SECURE/FUNCTION CHECK DONE OK.    HARIHARAN/SHAHRIE 31.05.2023 #3531 SMOKE DETECTOR NOTIFICATION 2493275