Pretty_little_liars_3x06

: 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.

: It also deepens the mystery surrounding Toby’s mother, Marion Cavanaugh, suggesting that the "accidents" at Radley were planned or covered up long in advance. Pretty_little_liars_3x06

In the Pretty Little Liars episode "The Remains of the 'A'" (Season 3, Episode 06), the "interesting paper" most likely refers to the found by Hanna and Spencer . : This paper leads them to discover that

Here is why that specific piece of paper was so significant: Pretty_little_liars_3x06