Didn't you wonder how the AAIB know that the last message wasn't read?
No, it never occurred to me to wonder that, I'm happy to accept that the network knows when any sent text has been opened by the recipient, and advised the AAIB accordingly.
That, of course, applies only to SMS/texts and not to IM/Instant Messaging. In the latter case there is no practical distinction between a message being received and opened - the clue is in the name, Instant.
