The built-in flash automatically fires even without being previously set. (EOS REBEL T5i / EOS 700D)

In the < > (Scene Intelligent Auto), < > (Portrait), < > (Close-up), or < > (Night Portrait) 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 of List of functions that can be set in each shooting mode (EOS REBEL T5i / EOS 700D).