If you look at the errorlog for a instance that has just started it shows something like

CHECKDB for database 'msdb' finished without errors on 2013-11-05 10:05:02.497 (local time). This is an informational message only; no user action is required.
CHECKDB for database 'model' finished without errors on 2013-11-05 10:05:01.953 (local time). This is an informational message only; no user action is required.
CHECKDB for database 'master' finished without errors on 2013-11-05 10:04:59.980 (local time). This is an informational message only; no user action is required.

You may think that this means that CHECKDB is being ran at start up on every database on the server, but actually its just reporting from metadata the “last known good” time for the database.

You can read when the “Last Known Good” is updated on Erins Blog

http://www.sqlskills.com/blogs/erin/what-checks-update-dbcclastknowngood

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