TIMS showed Smoke External Alarm at Front M2 car. Staff(AMK) checked and confirmed no signs of smoke/ fire / burning smell on exterior or interior of train. Re ar T258 line clear and confirmed no abnormalities. ~~~~~ Check and found #2604 external smoke detector to be very dusty. Replaced and function test -ok. Notification no: 2516011 Done by: Morgan and Haikel