No problem. If you're curious about service animals, there is an excellent site about them called "please don't pet me" which has an etiquette page.
http://pleasedontpetme.com/etiquette.php
It also has pages about what services animals can provide for those who need them(the dog doing laundry is adorable), and some info about the laws governing service animals. They aren't required to be in harness, but a lot of trainers use things like that to clearly indicate to the dog when they're "on duty" or off. This doesn't apply to a dog sniffing for ketosis, as they're on duty 24/7, but other animals sometimes benefit from the clear signal being in a harness(or a backpack) provides. They're trained to focus on their handler and their environment intently when they're on duty because a lapse like stopping to sniff something in the road or pick up a treat tossed by a well meaning stranger could literally get their handler killed if they stop at the wrong time.