The next page of our guide to Greedfall contains a detailed description of a Missing in Action companion quest commissioned by Kurt.
Where to start the quest?: Talk to Kurt when you get to New Serene and talk to Constantin.
Talk to Kurt and he'll tell you a story about a young rookie. Go to the barracks and talk to the soldiers about Reiner. Then go down the floor below and go to the morgue. You'll be stopped at the entrance to the room by a medic. Talk to the man and then try to use your charisma to get inside.
Look at the body and read the medical records. Then go back to the man and accuse him of lying. Return to the quartermaster, speak to him and start questioning the lieutenants. After that, go to the tavern, find another recruit and talk to him about Reiner. Buy a man a bottle of brandy (you can buy it from the innkeeper) and you will learn new facts. Return to lieutenant of 11th squad and get further information from him. You will be referred to another soldier - you can use charisma, bribe him, or let Kurt do the work. You'll gain one point of reputation with the Coin Guardians.
You will complete the quest - you will receive 1000 XP, the relationship with Kurt will improve +3 and you will start another companion quest - Amongst the Ghosts. To enroll in it, you will need to complete several quests of the main story first.