Use the Sardines to find the ring on the fountain. There is seagull perched up top of the fountain and won’t let you get near, so you need to distract her with the sardines. She’ll leave and when she does, you can click on the nest to find Fiona’s ring
How to Get the Sardines
The warden will shut the door behind you when you enter to the cell, trapping you inside. But lucky you: there’s an easy escape. Push the bed aside to reveal a hole in the ground. Go into the hole and you will discover a tunnel that the prisoner here dug with a pickaxe. Use your thermal scanner, and you’ll see the prisoner in the upper-right corner of the tunnel. Along the way you will find the Sardines.