Emails are sometimes mistakenly marked as spam or end up in the trash, making them hard to find using the Gmail search function. Google has changed the way Gmail search works, so it now searches in both the Trash and Spam folders as well as all your active folders. If a match is found in either of those folders, a message will appear in the footer at the bottom of the search results indicating the match and giving you a link to view those messages.

Another new footer message will be displayed if you enter a search term that’s too general to return useful search results. This message will provide guidance on refining your search query using Gmail advanced search.

You’ll find more links and a handy video at https://support.google.com/mail/answer/6593.