peeker noun a thief who operates by observing the numbers given at a cloakroom and then using a counterfeit check to retrieve valuable items that have been checked in US
[A] couple of peekers, suitably dressed for admission to one of the zizzier places, waits across the street from the nightclub entrance. — Dev Collans with Stewart Sterling, IwasaHouseDetective, p. 16, 1954