noun / verb
peep
PEEP/piːp/
A quick or secret look at something.
She peeps through a narrow gap in the curtains.

Etymology
Middle English pepen described looking from concealment, especially through a narrow opening—the image behind today's secret glance. It may be an alteration of piken, the verb behind peek.
In context
curiosity tempted me to creep along and peep in at the crack of the door standing ajar.