What Would Happen
If a service dog approaches you while wearing a vest, harness, or potentially a bandana, and has no owner in sight, that means it wants you to follow it. Service dogs are trained to come up and nudge you to get your attention, and then they want you to come with them.
There is no specific word you need to know to communicate with the dog either. Simply acknowledge that the dog is there, through some sort of verbal communication like saying “where?” or “what?” Trainers have also said if you just start moving, the dog will lead you.