To slip into or get mixed in with something else; to become lost among other things or people.
重要な書類が他の紙に紛れ込んでいた。
An important document had gotten mixed in with other papers.
犯人は群衆に紛れ込んでその場を逃れた。
The criminal slipped into the crowd and escaped the scene.
古い写真が箱の中に紛れ込んでいて懐かしかった。
An old photograph was mixed in the box and felt nostalgic.