to creep in; to steal in; to sneak in
泥棒が夜中に忍び込んだ。
A thief snuck in during the night.
猫が部屋に忍び込んでいた。
The cat had snuck into the room.
子供の頃に友達の家に忍び込む遊びをした。
As a child, I played at sneaking into friends' houses.