Thank you for your reply. But I think something is broken. All our posts have categories assigned to them. You should see the category the post belonged in the search results just under the featured image in small fonts.
Try search for something like credit suisse then unselect one of the category, you will see the total number of the results doesn’t change at all.
Could you look at it more closely to see what’s wrong? I think this happens after we need to re-import the settings, after we dumped the database from another wordpress.