The Observers

Sightings: (so far, links to YouTube clips for episodes 3x01-3x07)

Season 1:
"The Arrival" (an Observer centered episode)

Season 2:
"August" (an Observer centered episode)
"Peter" (takes place in the past with Observers in a non-observing role)
"Brown Betty" (a story episode, in non-observing role within the story)

Season 3:
3x01 "Olivia"
3x02 "The Box"
3x03 "The Plateau"
3x04 "Do Shapeshifters Dream of Electric Sheep?"
3x05 "Amber 31422"
3x06 "6955 kHz"
3x07 "The Abducted"
3x08 "Entrada"
3x09 "The Marionette"