Over the years, Outlook has widened its search scope beyond just the current mailbox.
On the search line, above a messages list, there’s a pull down list of options.
For mail folders, these are your options.
Current Folder – whatever folder you’re looking at
Subfolders – the current folder and any subfolders.
Current Mailbox – all mail items in the current mailbox
All Mailboxes – all mail accounts you currently have open in Outlook.
All Outlook Items – all items (mail, calendar, contacts etc)
Outlook defaults to searching ‘Current Folder’ but ‘Current mailbox’ when you’re searching from an Inbox.
But you can change the default to ‘Current Folder’ at Options | Search | Results
The Deleted Items folder is usually excluded from ‘All Items’ searches unless you choose differently on the Search option page. Deleted Items can be searched if you select that folder first.
The ‘All Mailboxes’ and ‘All Outlook Items’ search scopes mean you can quickly look through multiple mail accounts open in the same Outlook profile.
The term ‘All Mailboxes’ isn’t entirely correct it should really be ‘All Mailboxes and open PST files’. This scope also includes any PST data files you have open even though they are not linked to a specific mail account. For example, any AutoArchive or ‘offload’ PST’s you’ve created.