to disappear into; to slip into; to be lost in; to be mixed up with
偽札が紛れ込んでいた。
A counterfeit bill was mixed in.
野良猫が校舎の中に紛れ込んだ。
A stray cat slipped into the school building.
偽札が本物の中に紛れ込んでいる。
Counterfeit bills are mixed in with the real ones.