The purpose of this note is to raise awareness of how the software functions in the realm of private messages. A typical scenario would appear like this:
Person A receives a message from person B, and views the message. Person B can look in their private messages, and see that person A has read the message by an indication at the bottom of the message, which states "Unread By: none."
Now, person A has noticed their inbox is close to its message limit (or has reached that limit), and decides it is time to delete several private messages that have served their purpose, and are simply no longer needed. Person B now sees as the last entry of the private message they originally sent "Person A has removed themself from this topic."