Since having the firefighters called in would be a pain, Stiyl had written his orders for Innocentius such that it would not trigger the security sensors.

[...]

Given how long it would take for the firefighters to arrive, he could leave Innocentius on auto-chase, and the boy would get a nice flaming embrace that would turn him into black charcoal or white ash.

Toaru Majutsu no Index vol.1 ch.1 pt.7