(Using 1.2) I just noticed that when I import a mailbox, I can no longer look in it from the Unfiled smart category: I must either double click on it (opening a new window) or look for it in the folder hierarchy.
Could it be possible to have a disclosure triangle when mailboxes are added to the library and are unfiled?
I recommend instead that you select it in Unfiled and choose Select Source (or use the toolbar button) to select it in the source list.
I removed this in 1.2 because people found it confusing and because it was making EagleFiler a lot slower. However, there is a hidden preference to enable or disable showing disclosure triangles for smart folders.