TC reported smoke external at Rear M2 car. Staff(AMK) checked, no sign of smoke or fire at Rear M2 c ar. Rear T218 line clear, no abnormalities. ~~~~~ 2528 TC reported smoke external. Staff(AMK) checked no sign of smoke or fire at 2528. Rear T218 line clear, no abnormalities. CE: 2528 smoke external alarm able to reset.  2528 Checked found fault code 721 registered. Checked external smoke detector - OK, no water ingress. Plaseal, cable gland - OK. No gasket on terminal side of smoke detector. OPS 20 raised for smoke detector changeout.  Shaun 45299 BSD: 2528 To replace External smoke detector  Checked fault occurred once and resetted. Further checked smoke detector observed sensor slightly dirty and dusty. Serviced the smoke detector and function tested ok. Done by: Jack/Sanip 45301