That is like saying the clothes in your closet were mysteriously rearranged without you doing it or any possibility of anyone else getting access to your home, and then dismissing it as a fluke because it happened only once.
If what you say happened is true, the only explanation is that someone typed in that reply on the phone. There is no possibility of remote access control, or the phone self generating any response to messages. Even if you had previously created shortcuts to preset responses, someone would have to physically type in the shortcut in the reply box.
So, unless this event is the terminal process of the bovine digestive system, I certainly would not wait for it to happen again before figuring out who did it.