The white spot could just be the carapace naturally flaking off as is grows, or maybe not. If it's a pet store turtle that someone let loose, it probably won't survive in the wild on it's own. Twisterdog mentioned this earlier.

Your local or county animal control agency or ASPCA should be able to help you with a phone call. In the meantime, if you see turtles that look exactly like your lil' Orphan Annie swimming around the lake at the park near you, it's a good chance that's where he/she came from. An animal officer could tell you in a minute.

As far as eating goes, turtles will sometimes not eat for a few days, and longer in fact during hibernation season. Baloney is actually OK in small amounts (as a treat), but turtles become more herberific as they get older and should get more fruit & veggies.

If we were in the same area I would adopt the little guy. Please let us know what happens. You're very kind to spend the time doing this. It won't take much to see that this turtle survives.

If you have access to a camera, post some pics. Even throw away cameras give you a CD when you develop them. Take a couple turtle shots, and you'll still have 25 exposures for your other pets. Pets love getting their picture taken.