What do you know about guard dogs?

 Hmm, I've not seen them used in that way -- when business is open and customers are present.  I know of 2 businesses which used dogs to prowl the grounds after hours, and were crated during the day.  In one case, the dogs were truly vicious, had to be sedated to get them to the vet.  Even the owner would have been attacked if he got near them.  He had some way of working them into a small area and closing off a gate when he opened in the morning, which included raw meat.  In the other case, 2 dobies were used after hours to protect a fenced field area where cars were stored.  I phoned AC about that, as the dogs had no shelter, no water or food, only at meal times.  The owner could and did get along fine with those 2 dogs.  After the AC spoke to him about what he had to do, he decided to take them home with him and electrified the fence instead.

 With the dog present while business is open, I wonder if it IS a guard dog, or if they are relying on its size to keep folks in check.  I'd worry that some nut will consider the dog a challenge, and work out a way to harm it.

 Hmmm, I asked out of curiousity and was told it was a guard dog...but there was something in the gal's eyes that made me wonder.  Not sure I'd want to stick out my hand to let the dog smell me, though. 

 Your last paragraph is pretty much what I was thinking.  But you never know...

As someone who has multiple GSDs trained in protection, how I answer questions about them and what they do depends on how the question is asked and who's asking, if that makes sense. ;) 

The dog could very well just be a visual deterrent.  99% of the time that is all that is needed.

My instinct agrees with you, Leisje...but it still makes me hesitant to go back.  If I were the store owner, I would make sure the dog was out of sight, laying down or whatever...unless needed for protection...And that certainly wasn't me!  I wonder if he smelled my dogs, and popped up from behind the counter for that reason.