peep · verb I /piːp/
peep through/at/out, etc.
C2 to look at something for a short time, often when you do not want other people to see you
Dictionary example:
She peeped at them through the fence.
Learner example:
Once I peeped through the keyhole and I could see him crying in pain over my mother's photograph. (Certificate of Proficiency in English; C2; Spanish)