There's a story that went viral in 2018, but it's been a couple of years and I think it's time to revisit the possible situation.
A SERVICE DOG AND HER HANDLER ARE ALWAYS TOGETHER
A handler and her service dog are always together when in public. So, if you see a dog without her handler something is wrong. The dogs are trained to get help when the handler needs it. The handler may have had a seizure, fallen, fainted, is otherwise unresponsive, or actually asked the dog to go get help. Dogs aren't required to wear a vest or coat, but it's my experience that most do so that the public can easily identify it as being a service dog and so you don't approach it or pet it because it's working.
FOLLOW THE DOG
So, what do you do if a dog wearing a vest or coat that clearly states it's a service dog approaches you and it's alone? First, know that dogs are trained differently. There's no standard. It may sit and look at you. It may stand and nudge you on the leg. And, there's no cue for you to say or do. In other words, asking the dog, “what's wrong?” or “where's your mama?” or “show me!” may work, but may not. Simply start walking and follow the dog. She'll lead you to her handler.
The dogs generally have the good sense to know which person to pick out of the crowd – which person is going to respond and follow, but that's not always true. If you don't respond to the dog, it may go find someone else who will.
Just know that when you see a dog without her handler the dog is most likely not lost. It's being persistent on purpose. It's trying to get help for her person. Don't be afraid or if you are afraid summon someone nearby to follow the dog.
IT MAY BE TRAINING
This, “go get help” cue has to be trained. Scott and I will always go to the mall together to practice this cue, and Venus always finds him because she's looking for him, but I'm waiting for the day that she runs into another friendly face…maybe someone she knows, or can't find him and chooses someone else instead. Please don't be upset with the handler if you find her ‘faking it' while training her dog. The dog chose you for a special purpose and she thought you were a special person. Honestly.
IF YOU THINK THE DOG IS LOST
If you truly think you've found a service dog that is lost check the zippered pouches on her coat. That's where I store the ID card that has both Venus's name and mine and all the emergency contact information. If the dog isn't lost, and it is searching for help, this may also be helpful – especially if you can't find its owner.