The red tooltip you see is used to display validation errors in the MaskedInput controls. However, I am not sure why you get an empty tooltip in your case. However as we had issues with the validation errors tooltip in the past I want to ask you if you can test your solution with the latest RadControls for Silverlight version - Q1 2013.
If the tooltip is displayed in your scenario even when referencing the Q1 2013 assemblies, we will need more information about the definition of the control. If you can isolate the issue in a sample solution and send it over, we will highly appreciate it. Then we will be able to investigate the case further and suggest a solution.
All the best,
the Telerik team