Where were Levi and the scouts during the original attack in episode 1 when the titans first broke through the wall?
The scouting regiment was wiped out by the colossal titan before the attack on Wall Maria and those who were not eaten were too injured to fight, so they could not fight the titan invasion. During the fall of Trost, and the battle for Trost, the Levi squad and the Survey Corps were gone on an expedition outside of Wall Rose, finally returning just in time to save the remnants of the soldiers sent to help get Trost back from the titans.
Incorrect. The scouts pass through Shinganshina, after returning from their scouting expedition outside the walls, prior to the fall of the wall. We know this, because this is where Eren and Mikasa, as 10 year olds after collecting firewood, spot Commander Sadies and Erwin - aka the Moses scene. The Survey Corps have since left Shinganshina and are on their way back to Scout HQ, located at the old castle (s1) inside wall Rose. News of the fall would not have reached them quickly, as messages were delivered by horse and rider. Even if news was to reach them, they were in no condition to fight, and would no have made any difference to the situation, as the Armoured Titan would have broken through the inner wall by that time.
(Yes, I'm here 5 years later - not even ashamed. For all the people looking for info to clarify their location).