By default in SharePoint 2010, if you are using the search box on the top right hand side of a SharePoint 2010 sub-site, only search results for that sub-site are returned. If you want the search results to be returned based on the whole site collection, follow these steps:
On the top level site (Site Collection) click on:
- Site Actions
- Site Settings
- Under Site Collection Administration, click on “Search Settings”
- In the “Site Collection Search Dropdown mode”, select “Do not show scopes drop down, and default to the target results page”
- Click OK
When you now search from any site within the site collection, the results will be based on the whole site collection.