Prison Architect: Edition

Sometimes, I guess ramdomly, you will find a hole in the perimeter fence/wall surrounding your prison. I quickly found two holes behind cell toilets. Select the cell and click the "Register cell" option, eventually the guards will discover the tunnel.

Once you have done this you should go to "Demolition" in the "Construction" menu, there you will find the "Remove tunnels" button, activate it over all the tunnel (including the toilet boxes), wait for the workers to fill it and the achievement will be yours.

*Note that if you don't refill the toilet box this cell won't be operational since the game won't consider this cell closed by four walls anymore.
*I've done this in Prison Architect Mode, unlimited funds.

Tunnels can be found by searching Toilets, Cells, Cell Blocks, or by initiating "shakedown" from the "Emergencies" menu.

Dogs patroling the parimeter of your cellblock will also alert you to digging in that area with a brown "trowel" icon to indicate digging, dogs do raise false alarms when near machinery, so the best bet is to ignore any "digging" alerts during the day, and investigate ones at night, especially when clumped together.

Once you've searched the cell and uncovered the tunnel, select "Repair Tunnel" from the "Demolition" menu, the achievement will pop when all squares of the tunnel have been reapired.