Missing in Action is a quest received from the Chanter's Board.
The Chantry would like all travelers to be aware of a missing soldier, Rexel, and report any information discovered.
This quest cannot be completed until you visit Arl Howe's estate to rescue Queen Anora. Once there, he is located in Arl of Denerim's Estate - Dungeon. You get the key to his cage from the Jailer. This can be done before or after facing Arl Howe.