That's an interesting one, kind of a neat little visitor, though odd for him to like your shop, or it would seem.
If you seek a vet, make sure they are herpetologist friendly or deal with reptiles.
I've raised other species, since a kid, still do every year, small hatchlings, keeping them for the winter mid summer, release and repeat, kind a of a hobby, but my knowlege on turtles that stay mostly on land may be a bit limited and or a little generic.
There is a lot of information on the web about turtles, forums etc.
They are cold blooded, so that shop floor is probably cool and drafty, you should try to place him/her in some kind of appropriately sized enclosure and provide a heat lamp, to warm him/her up a bit, use a low wattage bulb, and make sure he/she can move away from it to regulate their body temperature, just enough heat to warm the upper shell (carapace) enough to get the turtle warm enough, to become more active, eat and drink etc. Not sure what food is best but you'll figure that out after identifying the species and doing some reasearch on the net.
Once you have observed the turtle eating, and leacving some scat, this means all systems are go, most species are hearty, if you have basic knowledge of their needs and provide it, they will thrive.
The painted turtles I have raised for so many years, get a lot of benefit from direct sunlight, not sure about other species, but that is hard to duplicate, requires a special bulb etc. Box turtle probably likes a more tropical environment, not so sure about shade or sunlight.
Have only seen one box turtle in the wild in my entire life, found one crossing the road in central NJ while headed to work, gave him a lift to safety and was on my way, kind of neat he likes the shop, for having a small brain, they are quite intelligent and seem to remember things.
I had a painted turtle come into my garage once during egg laying season, kind of a layover was 9PM or so, heard the clunking on the concrete floor, thought someone was up to something, could not figure out what it was, looked down and at my feet there was an old female, with mud on the fenders, smart move too, she'd have not made it back to the water in the dark and a coon would have likely found her, I dropped her off the next day. They often travel up the hill to lay eggs in the yard and surrounding areas, so I kind of help em out as a hobby.
One of the most interesting and rarely seen species in this area is the wood turtle, there is a clan of them down the stream a bit from here, was helping my neighbor load out 2nd cut round bales in August and found one, was injured and not doing well, I suspect the baler or something related gave it an impact injury, did not recover, they are similar to box turtles, don't have to feed in water etc.
