Dead drop

[1][2] The success of a dead drop depends on the location and method of concealment, ensuring retrieval without the operatives being spotted by the public, police, or other security forces.

In some cases, signals are made from an agent's residence, visible from the outside, such as distinctively coloured towel hung from a balcony or a potted plant positioned on a windowsill.

While the dead drop method is useful in preventing the instantaneous capture of either an operative/handler pair or an entire espionage network, it is not without disadvantages.

Counterintelligence can then use the dead drop as a double agent for a variety of purposes, such as to feed misinformation to the enemy or to identify other operatives using it or ultimately to booby trap it.

[4] SecureDrop, initially called DeadDrop, is a software suite for teams that allows them to create a digital dead drop location to receive tips from whistleblowers through the Internet.

This (replacement) mailbox is identical to the one, and in the same location that convicted spy Aldrich Ames used to signal his Russian counterparts. Ames would place a horizontal chalk mark about 3" long above the USPS logo.
Dead drop spike