Note: Though it certainly seems appropriate for Tagon and Bunnigus to take this conversation to a private channel, it's obvious that Danita can hear them. This leaves you, fair reader, with several possible interpretations of the scene.

• Danita's hearing is good enough that stray vibrations from the face-plates are audible. 
• Tagon and Bunnigus haven't figured out how to have private conversations.
• Tagon and Bunnigus deliberately had this conversation in a way that Danita could hear them.
• The Universe required a punchline (which it does, every day) and warped physics in such a way that the conversation was audible.

Pick whichever one of these feels right to you. Sometimes the Universe neglects to explain itself, and we need to tell ourselves stories so that it continues to appear predictable.