고 지나가다

do it briefly and move on

Means you do the thing and keep going rather than stopping: 인사 한번 드리고 지나갔습니다 "I said hello and moved on", 냄새만 맡고 지나가다 "just smell it and walk past". With 모르고 it becomes the standard way to say something slipped by unnoticed — 모르고 지나가다 "miss it without realising" — which two of these lines use. The similar-looking 게 지나가다 is different: that one describes how time or an event went by.