How the theme song was selected was brought up in a promo for the Pretty Little Liars special "5 Years Forward." In the promo, "little liars" Troian Bellisario, Ashley Benson, Shay Mitchell, and Lucy Hale are discussing the show alongside showrunner/head woman in charge I. Marlene King. As they reminisce about the early days of PLL, Troian mentions that it was Ashley who discovered the iconic tune: "I remember [Ashley] was playing it off of [her] phone, and was like, 'please, this would be so cool,'" says Troian of the awesome musical discovery. It turns out that Ash had great taste, because the network approved the song and it became the series' theme.
So how did Ashley discover the song in the first place? One potential way may have been from hearing it on that other show about an anonymous stalker threatening to reveal everyone's secrets. Gossip Girl also featured "Secret" in an episode, and it was as delightfully creepy then as it is now: While Gossip Girl certainly made this song work, it's hard to associate it with any TV series other than PLL. This song may not have been written for PLL, but it fits so perfectly among the mayhem of Rosewood that it may as well have been.
Check out the preview for the new special below: