In Messages on the Mac do you have the Preferences > Messages section set to "Save History when Conversations are Closed" ?
This will store all IMs and iMessages in ~/Library/Messages/Archive.
Use the Finder > Go Menu > Go to Folder and type that path into the Dialogue box that appears. (~/Library/Messages/Archive).
They are stored in dated folders so some guessing/trail and error will be needed.
You may find that if this was an iMessage and you send the person a new message that the whole History with that Contact since you installed Mountain Lion/Started Messages may appear and that it is scrollable the closer you get to the "top" and the top may keep moving until all is on show.
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously