Missing Souls

 is a Side-mission in the Red Dead Redemption Undead Nightmare DLC pack. This 'mission' is essentially a cutscene that initiates the Missing Persons activity.

Story
Inside Fort Mercer, Marston encounters the American Army Captain putting up a poster. Thinking that the Captain has been given the work of underlings, Marston jokes with him about it.

The Captain rounds on John, tersely pointing out that the posters are of people who are missing throughout the frontier and need to be found before they meet a grisly fate. Seeing that it is not a laughing matter, John decides to help rescue the missing persons.

Walkthrough
This series of missions will become available once all of the towns in New Austin have been rescued from the Undead.

Missing persons posters are found inside the strongholds of Fort Mercer and El Presidio.

There are 10 Missing Persons to return to Fort Mercer:
 * 1) Millicent Waterbury - Pleasance House
 * 2) Mintie Cummings - Riley's Charge
 * 3) Winona Picket - Lake Don Julio
 * 4) Grover Boone - Venter's Place
 * 5) Silas Gaskell - Critchley's Ranch
 * 6) Willis Lassiter - Benedict Point
 * 7) Chu Fook - Gaptooth Breach
 * 8) Orison Pratt - Ridgewood Farm
 * 9) Willie Oats - Plainview
 * 10) Adrienne Lachance - Armadillo

There are 6 Missing Persons to return to El Presidio:
 * 1) Porfiro Gutierrez - Roca de Madera
 * 2) Ramon Alvares - Torquemada
 * 3) Lydia Morales - Las Hermanas
 * 4) Rafael Carillo - Agave Viejo
 * 5) Santiago Valenzuela - El Matadero
 * 6) Merche Coronado - Casa Madrugada

Glitches
Sometimes during the cutscene, the man is invisible.

There is a glitch during the Missing souls mission when getting Milicent Waterbury. If you beat that mission then save, sometimes the missing souls mission will still be on your map, but checked off in your stats. The problem is if you go back to the Pleasance house where you get her, you must kill all the zombies again. Once the cut scene is over she disappears. There has been confirmed reports of not being able to finish all the survivor missions (get the poster, return the person) which makes it impossible to get 100%.

Achievement/Trophies
Completing all 16 of the Missing Persons missions contributes to the following trophy/achievement:

Almas perdidas