Outlook search: what are the rules???

I have a mailbox which I'll call XXX, which contains several mailboxes XXAAA, XXXBBB, etc. The mailboxes collectively contain dozens of messages, in some cases including several from the same sender., which may be in different boxes.

My task is to move all but one message from each sender to a specific mailbox, Duplicates. The one message not moved to Duplicates stays wherever it is.

The procedure is to search XXX for the sender of the first message I see in XXX. I know from experience that when Search in is set to "Current mailbox" it actually searches the current mailbox and all mailboxes contained in it, so it should find all messages from that sender in all of the XXX... mailboxes . I choose the message that I want to leave in place, then select and drag all of the others to Duplicates. Then I repeat for the second sender, and so on. When I have checked every sender visible in XXX, I do the same for each sender in each of the other mailboxes except Duplicates.

I know from experience that when Outlook shows a folder that is not in the selected mailbox, it displays the mailbox name along with the sender and first few words of the body. This is an important aid in making sure that I am handling each group of messages correctly.

But now Outlook is not behaving as I expect. when I search for a sender it lists messages in mailboxes that aren't related to XXX -- in Drafts, for example. And it doesn't show what mailbox a message is in, no matter where it is. This makes the display from the Find box just about useless, and makes it almost impossible to perform my task.

Why is Outlook behaving this way? How can I make it stop?