Dead-drop

The agent had to dead-drop the locker key to the PGU by some simple means, such as by taping it underneath a predesignated park bench, where it could be retrieved unobtrusively, usually by an officer under illegal cover.

Source: wiktionary

Last week, he 'dead-dropped' some pictures of the last dozen or so relatively new arrivals to Moscow and their assigned cars.

Source: wiktionary

Upon arriving in Setauket, Roe would dead-drop the information in a box in the middle of a field owned by Woodhull. Woodhull took the secrets to Brewster, who delivered them to Major Tallmadge.

Source: wiktionary