I saw her peeping through the curtains/into the room
The door was ajared and Helen peeped in.
. Don't peep! Keep your eyes closed or it'll ruin the surprise.
I thought I heard a peep.
One more peep and I'm gonna paste you right in the face!
She would have liked to peep in, but could not do such a thing.
If I hear one more peep out of you I'm sending you to your room without supper.