The speaker remembers a girl, addressed as "Rebecca," whom they once noticed but never spoke to. They meant to say hello and backed away, without knowing why. She seemed troubled, as if her mouth were "stitched shut," and the speaker recognises that feeling from having been in that place themselves. Then she vanished, and the speaker never learned where or why.
What gives the poem its ache is the speaker's regret. They keep looking for her face in crowds of uniformed people and "named strangers," and the repeated line "I still search for your face" shows the habit hasn't faded. The speaker has guesses about what happened, but none feel right. The poem ends by insisting, almost defensively, "I did care," even though they admit they were only a stranger. The mood is wistful, uneasy and quietly guilty, full of the pain of a kindness that never got offered. The uniforms suggest a school setting, though that is an inference from the imagery.