Large orgs could have a central knowledge base of all places someone could type a password (as required by code reviews, and perhaps by automated detection of password-type inputs), and a bot which attempts to fill correct and incorrect passwords for a number of canary users, and scans across all logs for plaintext canary passwords. It's shocking that someone like Facebook, which has so many partner-facing and user-facing interfaces, wouldn't have developed something like this.

Free startup idea: build a SaaS for the bot phase of this, and make the log-scanner simple and open source so enterprise security teams can deploy it freely. Offer a certification process and consulting. Hire lobbyists (and folks with inroads into insurance companies) to paint plaintext passwords as the devil and make yourself a de facto legal requirement. And make sure your log-scanner doesn't do any logging of its own :)