You have figuratively (not literally) been caught in a live mic situation. In the literal case, you think you are talking privately, but you are talking into a microphone that is switched on.
Here is an example of a literal live mic situation, from Paste Monthly: The Rivalries Issue
he [Biden] was heard on a live mic calling the Affordable Care Act “a
big fucking deal.”
Another example of being caught on a live mic, from the Washington Post
When Obama gave a joint address to Congress three months later,
Boehner and Biden were caught on a live mic before the speech
discussing the speaker's sub-par round of golf during the August
recess at a resort course in Nebraska.
This was only slightly embarrassing, because it was only about golf. However, the participants in the game had agreed not to publically discuss their scores at this "golf summit".
In the OP's example, after D had chimed in, A or B might say "Looks like we are on a live mic!"