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The technique described involves using physical symbols (markings on a stop sign) to convey a hidden message. This is a classic example of a visual semagram, a type of steganography where the secret message is hidden within the appearance or arrangement of physical objects. The symbols themselves are the message, hidden in plain sight from anyone not aware of their secret meaning. The communication's existence is concealed because the symbols would appear innocuous or random to an uninformed observer.

A. Text semagram: This technique hides a message by altering the properties of a carrier text, such as changing font sizes, spacing, or using specific word patterns, not by using physical symbols.
C. Grill cipher: This is a transposition cipher (a cryptographic method) that uses a physical template (a grille) to encrypt or decrypt a message hidden within a block of text.
D. Visual cipher: This is a general term for an encryption method that uses symbols or images (e.g., Pigpen cipher). It focuses on transforming a message (cryptography), not hiding its existence (steganography).

1. Wayner, P. (2009). Disappearing Cryptography: Information Hiding: Steganography & Watermarking (3rd ed.). Morgan Kaufmann. In Chapter 2, "A History of Steganography," Wayner discusses semagrams, describing visual semagrams as messages conveyed through the arrangement of physical objects, contrasting them with text semagrams which use the properties of the text itself.

2. Johnson, N. F., Duric, Z., & Jajodia, S. (2001). Information Hiding: Steganography and Watermarking - Attacks and Countermeasures. Springer Science & Business Media. Chapter 2, "Steganography," defines semagrams as a form of steganography that uses a cover that is not a digital file. It distinguishes visual semagrams (using physical objects) from text semagrams (modifying text appearance).

3. Kessler, G. C. (2001). Steganography: Hiding Data Within Data. Department of Computer and Information Sciences, University of Detroit Mercy. In Section 2.2, "Steganography Categories," Kessler describes visual steganography (or semagrams) as using physical objects to convey a message, providing the example of the placement of a postage stamp on an envelope. The scenario with the stop sign is a direct parallel.