We have more evidence of our past coverage that Google sends out notifications to Webmasters when they violate Google's terms of service. For example, Gabs received a notification and posted the details at Search Engine Roundtable Forums, explaining he was accidently hiding text on the page.

Here is the email he received:

Dear site owner or webmaster of *******co.uk,

While we were indexing your webpages, we detected that some of your pages were using techniques that were outside our quality guidelines, which can be found here: http://www.google.com/webmasters/guidelines.html
In order to preserve the quality of our search engine, we have
temporarily removed some webpages from our search results. Currently
pages from *****.co.uk are scheduled to be removed for at least 30 days.

Specifically, we detected the following practices on your webpages:

* The following hidden text on ********.co.uk:

e.g.
**********************
…

We would prefer to have your pages in Google's index. If you wish to be reincluded, please correct or remove all pages that are outside our quality guidelines. When you are ready, please visit:

https://www.google.com/webmasters/sitemaps/reinclusion?hl=en

to learn more and request a reinclusion request.

Sincerely,
Google Search Quality Team

Our past coverage of this includes:

• Google Sends Warnings to Sites
  
• Google Notification of Site Removal
  
• Google Steps Up Webmaster Spam Warning Emails
  

Forum discussion at Search Engine Roundtable Forums.