ehusted,
Let's just help you clear some logic hurdles that you may be stumbling over. Everyone who knows the system well already knows that it is an unassociated mailbox leaving at least one of the messages so I will help you understand how we get to that.
1. When a message is left for a user there is typically [2] lights that flash on the 5340; regardless of where it comes from. The ring indicator and the message notification light should both be flashing when they get the message on the phone, but both of them can be reprogrammed to do something else and may not be working as expected. The default feature code for Message is 365, but you can look it up here. System > Devices and Feature Codes > Feature Codes > Message.
2. When a password is input incorrectly three times it will send the user another message to let them know that someone has input a password incorrectly three times. This way they know someone is trying to "hack" their mailbox and should change their password immediately.
3. Looking under the Mailbox portion of the user's phone only shows their own mailbox. Mailboxes from other nodes will not show here, but they are still capable of sending the user messages as long as the phone exists within that node as a remote user.
So the next time they get the message you have them dial 365, or whatever the Message FC is, on their phone while it is idle and it should show where all the messages are coming from. Then you can find that mailbox and figure out why that user is the one being notified. They may need to use the Special Key if they are unable to dial the Message FC without getting an error; this depends on how your system is programmed. Alternately you could change their Message button, the one that looks like a flying envelope, to be Message FC once again and they can press that. Pressing it multiple times will show where other messages are coming from if the count is not the same as the original; ie you could have 8 messages from mb 1234 and 2 messages from mb 5678 when it originally showed that you had 10 messages. To make sure this happens you can leave a message in the user's actual mailbox and then you know there are at least two locations that will be found by the message button.
Personally I would make a button on the phone a message button; anywhere is fine if you don't want it to be the flying envelope. This will make it easier to work with and then you can delete it later.
Thanks,
TE