I agree. It probably wasn't a legit service dog. They go through very rigorous training, and would never have been certified if they had aggression issues.

Just to clarify: the reason you don't pet them isn't because they might be mean, it's because they are working and are required to be focused on their owner. Even if the dog is lying quietly on the floor next to his owner (or asleep) they are working.

If a service dog has his vest off (or in the case of a guide dog for a blind person, if he has his harness off), then you may appropriately ask if you can pet or say hello to the dog.