@cookpeter Welcome to the forum!
I moved your post into an existing feature request thread. To support this feature, please scroll to the top of the thread and click the Vote button.
A work-around for your use-case would be to create a dummy login item in your vault, save the leaked password in the password field of the dummy item, and then go to the Web Vault and run a Reused Passwords Report — this will pull up your dummy login item, as well as any items that have a matching password.