A thorough search of Gmail typically requires three steps:
1. Keyword search of your inbox by entering search terms into search box at top of page.
2. If not found, then opening Spam folder by finding it in left-hand panel (or entering in:spam in search box) and performing same keyword search again.
3. If not found, then opening Trash folder by finding it in left-hand panel (or entering in:trash in search box) and performing same keyword search again.
Now, steps two and three are but a click away if your search comes up empty in step one. If a message in either Spam or Trash matches your search query, you'll see a line at the bottom of your search results:
Some messages in Trash or Spam match your search. View messages.
If you click the "View messages" link, you'll see messages that you may have accidentally trashed or got caught by Gmail's spam filter. The messages listed are tagged to show you whether they are in the Spam or the Trash folder.
Also included in this update is help for overly general searches. If your search term is too general to return useful results, you'll see this message at the bottom of the results:
Improve result with with search options such as sender, date, size and more.
Clicking the "search options" link will open a panel that will help you refine your search terms to narrow the results.
So, the next time you can't find what you are looking for in Gmail, scroll down to the footer of Gmail's search results to stay on the scent by either searching your Spam and Trash folders or narrowing your search parameters.
This update is available only when accessing Gmail on the Web. Unfortunately, it is not available on Gmail's mobile apps.