You might have to be somewhat selective though. Don't tell them that you can or have mined the passwords (some of them might decide that you 'hacked' them). Maybe point out that potentially sensitive files are popping up on google, and try and look as dumb as possible. Don't let on that you're automating this in any way. Having said that, I'm sure that there will be a number of admins and users that will be extremely grateful for this!

I've sent similar emails in the past, and where you don't always get a reply spouting thanks (or a reply at all), you will likely notice the file not being available anymore.