TIMS showed Smoke External Alarm at car2523. Staff(CTH) checked and confirmed no signs of smoke ext erior of train. Rear T124 line clear and confirmed no abnorm alities. TIMS showed Smoke External Alarm at car 2523. Staff ~~~~~ Check rooftop open ltatch cover found smoke detecter with water ingress. Replaced new smoke detecter ok. Function check ok. Monitor 1/2hrs check ok. Rooftop ltatch cover fix with QC check ok. Tims show fault code 721.  Ram / Firdaus / Sridhar 02.12.2023