Why would it be so much surprising to see a guide dog without its owners?

For one, the dog and the handler are usually not apart when in public. Think about it for a minute----what use is a Guide dog for the handler if the dog is not with the handler? Most Guides will not leave their handler even if the leash and harness handle are dropped. That is their person, and they do not normally leave their side. The only time my wife's Guide is away from her in public is if the dog has to relieve and I am taking it out to do so or back in to where she is waiting.
If something has happened to the guide dog handler, the dog may be searching for someone to help their handler. My own service dog is trained to do this.