Depends, how did they get to the server (secure jumpbox)? Did anyone know it'd be a security related event when they were diagnosing the problem? Did the tech take precautions? In such a case, I don't think robinhood.com would need to notify users. They tcpdump would be handled securely, scrubbed properly, troubleshooting done, and that'd be the end of it.

If that isn't what happened, someone should be losing their job. The minute you think diagnosing network problems is a reason to ignore security to a degree that your users need to be notified you're incompetent.