OS X :: Trashed Item Unrecoverable On IMac

Unlike many other issues involving the inability to trash, or to keep an item from re-appearing, my iMac had just developed a quirk that is concerning. As with my Macbook pro, I have always been able to drag an item(s) to the trash, and until I empty the trash - to see those items and even drag them out. Now I still can do that with the Macbook, but not the iMac. As soon as I drop an item in, I get a msg asking me if I want to delete this item, as it will be unrecoverable.

Sure enough, drop drag and drop it into the trash and it disappears - permanently, no sign of it at all in the trash. No, the box is not ticked in the Finder preferences, to Empty Trash Securely. I do back-up, but only at the end of the day, and dropped a music file into the trash accidentally, thinking it was an unwanted copy - and it was very much wanted. That's when I discovered this new quirk. Why is this so? iMac - 3.06GHz Intel Core 2 duo; 4GB SDRAM; running 10.6.4. I am running 10.6./4 on the Macbook pro as well.