Which app sent the unauthorized texts?
If you go through your apps list you can see all the apps that have permission to send texts (or you could use appops to save having to click on every app), you can then narrow it down based on how reputable the app is, and/or doing web searches, once you thing you have narrowed down the app you can either, uninstall the app or block the permission using app ops, however if it was text secure that was the offending app, then you wont be able to tell as it has already been uninstalled.