Expect-CT Reports can be setup on your site with a single line of code or config and you should always hope to receive no reports.
A single HTTP Response Header is all that is required to enable this powerful monitoring.
If a user is receiving a browser warning and unable to access your site, the browser will now dispatch a report to let you know.
If this is reliably happening on your site, and you're receiving a large volume of these reports, it indicates a big problem for your users.
Alongside enabling Expect-CT Reports, you may like to further improve your security.