In the Star Trek: Deep Space Nine episode "Improbable Cause," we see the Changeling head of security Odo meet with some unknown Cardassian informant. Since that episode, there have been plenty of possibilities as to who that was, but as far as I know, there is no confirmed answer within the show. Has this been answered out of universe? Was this supposed to lead to a story arc?
In the seventh season episode Treachery, Faith and the Great River, Odo told Kira he was going to reach out to a Gul Russol , who'd in the past informed for him- unfortunately said Gul died before Odo could meet him, likely purged by order from his Dominion masters.