I did in fact have the Verizon Message App installed as well as the default message app. When I uninstalled the Verizon message app, the problem persisted. However, in the default message menu, ... (3 dots on bottom while viewing message list)..I previously only had a "settings" option. The rep walked me thru that process and that was the only option available. Later in the day, a "delete all threads" option appeared. By deleting all threads, the messages disappeared and the problem has not happened again. I'll just attribute this to a minor glitch if you will, but, it definitely seems to be related to the Verizon Messaging App. That app is nice but seemed to be the cause of the undesirable activity.