This post analyzes some of the communications techniques employed by characters in season 5 of the series "Bosch" on Amazon Video. By way of background, "Bosch" is a crime series based on novels by Michael Connelly depicting LAPD Detective Hieronymus "Harry" Bosch.
In this season, Bosch is investigating a murder suspect, who is also a shill for an opioid pill mill. Bosch goes undercover as opioid user 'Dominic Reilly', while his partner Jerry "J." Edgar and another detective, Rene Davila, surveil him.
Bosch and J. Edgar have pre-arranged "Hold Fast" as their Duress Code. This Duress Code is an example of a Covert Distress Signal, a pre-arranged method of conveying danger that is known only to privy parties. The code is a shared semantic token known only by Bosch and his partner J. Edgar. If Bosch were to encounter danger while he is undercover as Dominic Reilly, without giving up his cover he could use the phrase 'hold fast' somewhere in conversation.
The first use of the Duress Code is when one of the pill mill shills admits to Dominic Reilly that he (having spotted Detective Davila's car) believe they're under surveillance. Dominic offers to help get rid of the cops, and using the shill's cellphone, calls 911 to report an officer down at "Hold Fast Pizza". J. Edgar & Davila hear an all-call go out on the radio system for an officer down at "Hold Fast Pizza" and know right away that their cover is blown, so they bug out.
In this instance, the Duress Code "Hold Fast" was passed through the 911 PSAP & LAPD radio system, turning them into a Covert Channel for transmitting an encoded message. Because Jerry had visual on Bosch, and knew he was not in danger, hearing the Duress Code over the LAPD radio system indicated to him that there was some other compromise (their surveillance cover was blown).
Dominic befriends Lindsey, another opioid user, and while she's distracted he slips a note into her pocket (Brush Pass or Dead Drop). Later, while out on a pill harvesting run at a pharmacy, Dominic puts an item from the store into Lindsey's purse, and alerts a security guard that she has been shoplifting. The security guard apprehends Lindsey, and while the police are called, the remainder of the pill mill participants leave. After searching Lindsey, the police discover the note left by Dominic (Covert Channel), which contains J. Edgar's number, along with the Duress Code. The police call J. Edgar, who questions Lindsey in an attempt to locate Bosch.