Updated: Nov 9, 2018
Google Drive now allows you to limit a search for files to a single folder
Search Within Folder is very simple and powerful. Select a specific folder in Drive and limit the search to that folder and its subfolders.
In Google Drive:
Right-click on a folder
select Search within that folder
Only folders that are within your My Drive or Team Drives can be searched, if you have a folder that is shared with you, first add the folder to your My Drive.