Especially a service dog who isn’t just a companion but a helper.
Most dogs with a service vest are "emotional therapy dogs." Maybe the owner bought a certificate from some "therapist." Or the owner might simply have bought a service vest online.
Legally, stores are not allowed to ask for proof that a service dog is a real service dog.
Real service dogs are trained to be well behaved. But I've seen plenty of "service dogs" with vests in the supermarket that are anything but. It's obvious the owner just bought a vest online.