By default, WordPress pings pingomatic each time you publish a new post. Pingomatic in turn pings a whole host of other services, and Google will index the content and index it quickly unless you explicitly tell them not to.
If you don't want those pings to be sent at all, you can turn off pinging in Settings->Writing and removing pingomatic.com from the Update Services field.
In my experience, no web content is hidden or undiscovered. Google will find any pages published no matter if you use a sitemap or pinging services. A page will not be indexed, if you use meta on the particular page - <META NAME="robots" CONTENT="noindex.. etc robot.txt file is good for disallowing some crawlers.