I use Google Drive with multiple accounts. How can I search for something across all of these accounts?
Share the all the files and folders from the accounts that holds the files with the account that you want to use for searching.
- Use a folder as a "Master Archive", so its's sharing settings will be propagated to all the files and folders added or moved to it.
- Select one of your account as the "master account"
- Create a folder to be used as a "Master Archive" in your "master account" and share it with editor access to your other account.
- At each account move the "Master archive" of step  to "My Unit"
- Move or add your files and folders to the "Master archive" of step .
- Use a third-party app.