One is in the destructor of the statusHolder:

https://github.com/mongodb/mongo/blob/r3.1.9/src/mongo/db/s/move_chunk_command.cpp#L83

and the other is when the critical section finishes:

https://github.com/mongodb/mongo/blob/r3.1.9/src/mongo/db/s/move_chunk_command.cpp#L511

This creates a small window where a new migration can start and the destructor will set the state of the migration to not active.