Compromised prefix code
Transmission containing a compromised prefix code for the Titan
A compromised prefix code was a numerical code a Starfleet captain could give that would ping a designated starship, but also surreptitiously indicate that said captain had been compromised by an enemy.
In 2401, Captain William T. Riker gave a compromised prefix code after being captured by Vadic at Daystrom Station. When the Changelings attempted to use it to trace the location of the USS Titan-A, the code revealed to the Titan crew that Riker was being held aboard the Shrike. (PIC: "Dominion")