Real Estate Agents, under all circumstances and regardless of any fiduciary obligations to their client, must always disclose material facts. Indeed, real estate agents must seek out and uncover material facts so they can disclose them to all parties in the transaction. 
See previous posts on "Does Your Realtor Work for You?" and "Becoming a Client with a Realtor Agency Agreement" for background information prior to reading this post.
In "Does Your realtor Work for You?", the fiduciary responsibilities of a real estate agent to a client were outlined. If you are not the client, you do not benefit from the agent providing those responsibilities. Becoming a client is easy but does require some commitment. 
When someone is hired, they expect to get paid and generally speaking they are accountable to whoever is paying them. Real estate agents are different, they work for whoever signs an agency agreement with them which is not always the same person paying. And the distinction is not …