: This paper leads them to discover that a room was built into the sanitarium that wasn't on the official public records—a room that "A" (Mona) was using to sneak in and out of the facility.
Another notable paper trail in this episode involves the Hanna finds in Wilden’s car, which places him in a different location than he claimed during the time of the "remains" discovery. Pretty_little_liars_3x06
Here is why that specific piece of paper was so significant: : This paper leads them to discover that
: Spencer realizes the blueprint is dated months before the construction of that wing actually began. Pretty_little_liars_3x06