If you've kept your old emails around (woohoo gmail!), one thing you can do is go back and look for the notification email you got for a particular post that you suspect was a message. There's a date search widget that makes this easier. I suspect you'll find it was a wall post.
Another thing to note (for those that follow our technology), is that the backends for these two systems are entirely different. The messages backend is hbase-backed, designed for real-time communication more than history. The timeline backend is MySQL / C++ backed. Migrating data from hbase to MySQL would have to take months of effort.
I know that even though there was no bug here, this can be an alarming experience to see old posts surfaced like this. We're working on ways to make this interaction clearer so people aren't so surprised in the future. That said, please rest assured that your inbox is not on your timeline.