Ok, sorry, I jumped the gun out of panic (I was on the way to a long presentation when the user showed me his phone with the history of messages not related to him, and I posted from the coffee break).
Now that I checked myself, I see that the other user's message shown are all OLD ones; around 2009 - 2010, and not ALL messages in that period (or the history shown would be much larger). They are random messages addressed to someone else from the other user, along with actual messages exchanged with the current user.
So either something went wrong in that period, archiving some messages in a way they are now "public", or some weird database corruption made them visible to anyone. I cannot recall when I migrated the native database to MySql, but it may well be around then, so something in the migration could have messed with the archive.
Now, is there a way for me to make those messages "private" again without simply deleting all messages from that period? Is there a specific table/field related to which archived messages can be shown to whom?
Thanks again,
Joao