暮然回首灯火阑珊
[mù rán huí shŏu dēng huŏ lán shān]
'Looking back suddenly amidst faintly lit surroundings' derives from ancient poetry describing a scene of unexpectedly finding someone long-missed in crowded places under lanterns. Nowadays, this expression often alludes to reminiscing about or unexpectedly encountering the past amidst the hustle and bustle of life.