That could be a rogue app - one that's not supposed to be sending messages, but is. Finding it can take a long time. Start in Safe Mode. Run for about twice the time between messages. If there's no message sent, run an app and close it. Run the same amount of time, waiting for that app to send a message. Etc., etc., until you find the one that sends it. (It's probably an app you god from somewhere not the play store or Amazon.)