II.in [美 ɪn, 英 ɪn]副in often appears as the second element of certain verb structures in English (close in, hand in, stand in, etc). For translations, see the relevant verb entry (close, hand, stand, etc).
Any hidden location could serve, although often a cut-out device is used, such as a loose brick in a wall, a (cut-out) library book, or a hole in a tree.