Oops, I took the last online date as the last comment date, hadn't realised last comment prior to mine was 2 years ago.
Anyway, I did a little test with this issue, and I'm sure others are aware, but the message does seem to clear after I guess the engine has been off for a little while. This morning I opened the boot but only the drivers door, and there was no saved message. So I guess it's either a bug in the system or it's saved in the messages so the driver can check who is wearing a belt in the back during the journey maybe. Doesn't appear to be any solutions to this online, but doesn't seem a real problem.