unusal reaction to friends of our family

Question
We have a four year old male Bichon Frise who is absolutely delightful!  In fact, his name is Buddy because he is a buddy to everyone he meets. Well, almost everyone.  We have friends who are dog lovers themselves (they have a Golden) but whenever they come to our house, Buddy has the most bizzare reaction to them.  The instant one of them comes through the door, Buddy slinks down, tail down, hair stands up, ears down, then he softly growls and creeps over to me or my husband. He has never had a history of this behavior with anyone else. One or all of this particular family could spend hours at our house and Buddy would never "warm-up" to them.  Buddy has never been mistreated by them either.  We are at a loss here.

Answer
Buddy is responding to something (sensory) about one (or more) of these visitors.  His response is clearly fearful and avoidant.  This could be a response to something visual (a conditioned fear response built upon the behavior of one or more of them when, at the same time, something else occurred -- such as a sharp pain he had, a noise outdoors, etc.) or something he is scenting.  Dogs respond to the presence of illness in various ways; some alert, some retreat, some show aggression.  It's possible their Golden is suffering from an undiagnosed condition, or one of them is.  This is the best possible guess I can make from here and my advice is: when these people come to visit, confine Buddy to another room (well before they arrive) with special toys, a treat and a TV or radio playing, and leave him there until they leave.  Whatever this dog is responding to is real to him, not easily diagnosed, and definitely disturbing him.  So protect the dog.