If memory serves me right you need to find either the tunnel entrance or exit to uncover the whole tunnel.
Try searching the nearby cells or performing a shakedown on the whole cell block to see if you can find the entrance before they get outside the wall.
Be aware that this is not guaranteed to uncover all the tunnels according to the Prison Architect Wiki:
As of Alpha 34, manually searching a toilet will reveal any escape tunnels stemming from that toilet. This does not include searching a cell or cell block, but rather the suspected toilet itself. This does not get rid of the random chance that tunnels will be found on a blanket search, but now if you want the check a specific toilet, you don't have to dismantle it.
Just be careful with your prisoners as they seem rather unhappy already and the search is not going to help with that.