It should not be let go, unless you know exactly where it came from. Box turtles have a very strong homing instinct, and do not relocate well from their home range. They almost always end up dying trying to get back again.
The risk of salmonella from turtles is no higher than it is from grocery store bought chicken, or most people's kitchen sponges. Simply make sure you wash your hands thoroughly after handling, and don't let children stick their hands in their mouth, etc. Nothing less than you'd do if you were handling a piece of raw meat.
Box turtles are omnivores. Fruits, veggies, insects, and even some other meats. Chelonian Trust's Box Turtle Information
She sits in her corner, singing herself to sleep.
Wrapped in all of the promises, that no one seems to keep.