Specifically on Android when a message is sent the push notifications come through but if its not selected to open the messages or if the push notification just remains there for the user to see when they drag down from their screen... the app itself does not indicate any unread messages in the form of a red dot in the corner of the app nor does it indicate how many unread messages there are with a number.

Example in the attached shows the difference between Facebook messenger and the d6 app.