Give the agent a call.

If he brings an offer, all is forgiven until after closing! 

I had this happen once.  I received notification that an agent had accessed the lock box.  I called the agent and said it appeared that he'd been in the property.  He said he had.

Then, I said I didn't recall him contacting me for an appointment.  He stammered and said he thought it would be okay to go in.  I told him he thought wrong and to never do that again.

The unfortunate thing is that you can't teach this kind of person to be appropriate.  You might consider having a seller controlled lock box and have them set it out for confirmed showings.

I would give owners a chance to calm down, but also verify the scenario. i don't know if they had pets, or personal documents sitting out that they didn't want anyone to see, etc? Then have them writte up a complaint, deliver a copy to the agent and broker, but i would alert my broker before doing anything. Many times broker to broker communication can eliminate overreaction and get things solved.

If showing instructions were INCLUDED and CLEAR in the MLS I'd call the Broker or maybe even the Board.   "Horrified" is a big statement.

Start by giving the agent and their broker a call, could be just unaware, or could be criminal activity.

Yikes!  That was not cool.  

I would have a conversation with that agent and their broker.

I have had this happen and the seller was so furious (rightly so) that there was a police report filed.  One of the greatest acts of trust we are given is an owner providing a key to their home - that someone would violate this trust and someone's privacy is unforgivable.

Send an e-mail and cc his broker and yours.  Tell him if it happens again it will be reported to your licensing department.  Thats called tresspassing.

Call and find out the reason.   

I would let the agent know and that this agent should call or write an apology to the sellers.  Otherwise, reporting him might be in order.

It happens when property is popular and in demand. I will take care of it is what I tell the principals. As to the agent? Lets stay focused here on mission and not get into tangents. they take form our successes. Choose the battle

Call the agent and ask why? Of course, going there without an appointment is a no-no!

I'd probably let my broker know and let the other agent/his broker know what happened and that it should not happen again. I don't think I'd take it to the board the first time.

When confronted and without a sincere apology this would get you banned from any future showings of any of our listings. YOU'RE EXCLUDED!

I think I would have a conversation with the agent and put him/her on notice that it should not happen again.... I would follow that up with an email to his/her managing agent....that's as far as I would go with it....