Encrypted disk image no longer demands password

Some time ago I created an encrypted disk image for storing sensitive data. It has been working fine for months, everytime I clicked on it it demanded that I enter the password before it mounted. But suddenly yesterday it stopped demanding he password and would just mount upon clicking. Tried restart, no change. Tried repairing the disk permisssions, no change. In the meantime, I have created a new encrypted disk, moved all the info to that, and secure-trashed the old one. But this gives me pause for thought. Not terribly secure! Anybody know what might have happened? How can an encrypted disk suddenly become unencrypted?