A jail cell.
It is just before the second checkpoint. If you count platforms, including the starting platform it is the seventh and it's larger than the others. Once reaching this platform, jump to the staircase below and roll backwards down it. You will fall in a hole leading to a prison.
No investigation needed here.
This section is reserved for Confirmation/comments about this investigation. Investigations will remain open until advised by Redlynx that the final solution for each secret is found.