Ask about the child at the outpost.

Summary A child? Here? I am not following you. You'll have to tell me more details!
Come to think of it... I remember a child
who kept coming to visit the captured monster.
I warned the child several times that the monster is dangerous,
but he kept finding a way to the monster.
How did he find where the monster was every time?
The soldier over there will know more.
He was investigating the scene when there was an incident.

Quest Text We're still building the outpost.
Although it's still under construction, you won't have to worry about it collapsing.