The built-in flash automatically fires even without being previously set.

In the < >(Full Auto), < >(Portrait), or < >(Close-up)mode, the camera automatically judges the shooting environment to determine whether the built-in flash fires or not. Therefore, the built-in flash may automatically fire in low-light environments even if you do not want to use it.

To prevent the built-in flash from firing automatically, change the shooting mode.

For the details, see the built-in flash section at List of functions that can be set in each shooting mode (EOS 60D).