Sophist, why do you highly doubt this ornate box turtle is indigenous? I grew up in Southern Missouri and saw many, many, many turtles exactly like this one. It is naturally found in many states in the wild. Eastern box turtles are more common captive bred pets, not as often ornates. Her hibernation spot may very well have been disturbed, and she was out and about seeking another one. The longer she is kept, the greater chance of her not making it. Her shell is also a bit beat up as is common in a lot of wild turtles who have been bumped by cars or whatnot before. If she is being kept inside, goes from cool outdoors, to warm indoors, back to cool outdoors, that alone could cause her to sicken and die. What region do you live in M&M's Mommy? It would be very easy to find out if ornate box turtles are native to your region.

Caution: ornate box turtles are not hardy and are not suitable for beginners. The have very specific needs, are very sensitive to stress and are difficult to keep in captivity. The wild capture of ornate box turtles for the pet trade has caused the untimely death of many of these easily stressed and delicate turtles, so if you are determined to keep an ornate box turtle please seek out a captive bred specimen and do not support the capture of wild turtles for the pet trade.

Additionally, box turtle populations are declining (listed by CITES as threatened, and import/export permits are necessary). Many states protect box turtle populations and have laws against collecting box turtles from the wild. It is best to get a pet box turtle bred in captivity from a reputable breeder. Wild caught turtles do not adjust well to captivity and many die from the stress. Pet stores often carry wild caught turtles, so be careful.

link to article

edit: Also it does not matter if your back yard is fenced, turtles are excellent burrowers and climbers. They can easily get in, and out, of a fenced yard.