In Microsoft Word, if you want to search for Returns or Tabs in a document, you can do so by inserting a ^p (Return) or a ^t (Tab). But in Pages, Apple recommends that you select those characters from the Find dialog box’s Advanced tab. If you tried to press the actual Tab key in the search field, you’d just move the cursor to the next field; if you typed an actual Return, you’d activate the Next button.
You can, however, insert those characters in a Pages search field from the keyboard. To search for a Return, enter Option-Return in the search field; to look for a Tab, enter Option-Tab. For those and other special characters, you can also select an example of the character in the body text and then copy-and-paste it into a search field.