First of all, make sure you have the latest release available. In the past there were many bugs which allowed SQL injections. There were no "occurrences of lost posts", but somebody may use an old bug.
Second, always do back-ups of your database. Then intrusions like these won't be so critical.
Third... miniBB doesn't log any information regarding topics deletion. It's mini. If you assign moderators, make sure you do this for proper people. It's a very common administration mistake when full mod rights are assigned to the person you don't know personally.