Exam 312-49v10 topic 1 question 240 discussion

Harold is a computer forensics investigator working for a consulting firm out of Atlanta Georgia. Harold is called upon to help with a corporate espionage case in
Miami Florida. Harold assists in the investigation by pulling all the data from the computers allegedly used in the illegal activities. He finds that two suspects in the company where stealing sensitive corporate information and selling it to competing companies. From the email and instant messenger logs recovered, Harold has discovered that the two employees notified the buyers by writing symbols on the back of specific stop signs. This way, the buyers knew when and where to meet with the alleged suspects to buy the stolen material. What type of steganography did these two suspects use?