11-digit Message appears then disappears
Over the last couple of weeks I've had strings of digits appear in Messages, with no sender. The last one was an 11-digit number that I managed to copy down before it disappeared later in the day. I had thought it was a phone number, and wondered how the message could have been added to the message list when it came in, since it had not triggered a visual, text, or audio alert, which I've set all incoming messages to do. The string of digits is too long to be a phone number, as was confirmed when I entered it in the Who Called Me website. I checked the time stamp, and this message arrived while I was using the iPad, but no alert was triggered. Thinking that iCloud was simply syncing my Apple devices, I checked the Messages apps on my other Apples and found no such number on my iPhone or iMac. Had it already appeared and disappeared?
I have a wild guess....this number is a verification or validation probe that's sent periodically to check on the status of my iPad (and maybe my other Apple devices, too), and it's not supposed to be seen. Could be a glitch with the upgrade to iOS7 or OS X Mavericks. I don't know how to examine the properties of iMessages, so I don't know if it's an internal communication or came from outside my home WiFi network (my iPad is WiFi-only).
Any ideas? I haven't displayed the number I copied, since it may be an identifier of my iPad or, shudder, of me.
OS X Mavericks (10.9), iMac•AppleTV1•iPhone5S•iPad2•iOS7