TIMS showed Fire External at RDT car. Staff(AMK) checked and confirmed no signs of smoke / fire / bu rning smell on exterior or interior of train. Rear T412 lineclear and feedback no abnormalities. ~~~~~ 3517 TIMS showed Fire External. Staff(AMK) confirmed no signs of smoke/fire/burning smell on exterior or interior of train. Rear T412 line clear and feedback no abnormalities. CE 3517 fire external alarm able to reset. No abnormality. Depot 3517 smoke detector operational check.  3517 FC 721 (smoke external). Checked and found #3517 external smoke detector no cable gland. Presence of gasket on plastic cover. Ops20 raised to replace external smoke detector.  Aliff/Daniel 45218 BSD to install 3517 external smoke detector cable gland.