Tortoises are good at escaping and, surprisingly, climbing!
Can you get your arm under the shed and feel around? What's the area around the shed like, can he get through to the other side of it and into open space? Is the shed area good at water run off or does it sit in a bowl and flood? Do you have a dog?
I don't think an eagle, a good Scottish eagle, will see a tortoise and think food rather than rock. A cat might possibly play with it but I can't see it being dragged off and scooped out. Put food out and keep an eye out I think it'll turn up.
Our garden is sealed with old roof tiles, brick walls and paving slabs after an escape many years ago. You can pen them in somewhere with a run made of boards, just have open space, some shady area, shelter, water and food, plus grow weeds and nasturtiums. Ours like nastyurchins very much!