Grand Unifying Guesses/Torchwood

Torchwood 4 is missing because they are...

 * In the digital world.
 * Dead in the Underground.
 * Alternately, they are down in the Underground. They'll find someone true. Down in the Underground, a land serene, a crystal moon. Yeah. Hah.
 * Stuck traveling with a timing device.
 * Pulled back in time to help the Minbari. Duh.
 * Lost in a Sea of Dirac.
 * Trapped inside a Shield World's dyson sphere.
 * Just a code named for a subculture of Americans who hunt dangerous paranormal creatures.
 * Lost in London Below.
 * Lost walking the Silfen Paths.
 * On the Island.
 * A failed branch of the Special Talent Project
 * Were inside of Sunnydale High in '99.
 * Hogwarts.
 * Contained by the SCP Foundation after getting entangled in a particularly convoluted Temporal Paradox.
 * In Northern Ireland. And nobody cares about them.
 * In Narnia.
 * Trapped on the Game Grid.
 * In the middle of the desert in South Dakota.
 * Somewhere, over the Rainbow.
 * Crossed through the gap in the Wall, wandering the Realm of Faerie.
 * Swallowed by a Crack in Time.
 * In Canada, no one's just ever bothered to check.
 * Hidden in a small town in Maine.
 * In Portland, Oregon, staffed by a homicide detective and a reformed werewolf.