TIMS showed Fire External Alarm at car 2664. TC checked and confirmed no signs of smoke / fire / bur ning smell on exterior or interior of train. Rear T203 line clear and confirmed no abnormalities. ~~~~~ TIMS showed Fire External Alarm at car 2   #2664 found smoke external detector unit to be dusty and water ingressed. Replaced and function check ok. Notification to CSC: 2577080 Done by: Din, Morgan, Kumar (5/3/24)