The Spiral symbol has now been seen several times on Teen Wolf. Derek Hale explains to Scott that it is a symbol of revenge among werewolf packs.
The first appearance occurred during Second Chance at First Line. The spiral appeared around Laura Hale's grave as Stiles removed a wolfsbane studded rope, next to Derek's house.
The second appearance was a glowing spiral on the roof of the Video 2*C near the beginning of The Tell. As Derek and Scott leave the roof, the spiral becomes visible, glowing red.
The third was during Heart Monitor after the Alpha chases Scott McCall outside the Argent home. The Alpha traces a spiral on the window of Scott's Mom's car.
The fourth appearance, also during Heart Monitor, comes when Derek confronts Scott's boss about a spiral symbol found on a dead deer.
In Season 2, the spiral appeared in one of Lydia's hallucinations during Venomous. She sees Peter Hale writing out an equation on the blackboard in the shape of the spiral.
In Season 3, the spiral made another appearance in Visionary. Ennis once clawed a spiral into a metal wall of an abandoned Beacon Hills distillery. It also appeard in The Girl Who Knew Too Much, when Cora scratched it into the glass window before she attacked Aiden for helping kill Boyd.
Derek has a triple spiral or Triskelion on his back. It is three spirals connected together and connected to The Hale Family.
The triskelion is an ancient symbol with wide and varied meanings and usage around the planet.