This is really awkward because the full path isn't visible in the search results. For a fun time, type "resource" in the filter bar in the 2nd level inspector, for example.
<rdar://problem/19796772>
This is by design and explicitly supported. See WebInspector.ResourceTreeElement: get filterableData() { return {text: this._resource.url}; }, Delete that getter and it would do what you are expecting. However, I would argue matching the whole URL is useful. For example, when searching for a domain like google.com.