Lucy Fletcher suggested that the Ghost's equivalent of the Bat-Signal was an app. (TV: The Return of Doctor Mysterio [+]Loading...["The Return of Doctor Mysterio (TV story)"])
When Rhys Williams invited Jack Harkness, Gwen Cooper and the rest of their friends to Badger's house for a barbecue to remember their recently deceased friend, Dav, he claimed that he'd "sent the bat-signal", in reference to the invitation. (AUDIO: Rhys & Ianto's Excellent Barbecue [+]Loading...["Rhys & Ianto's Excellent Barbecue (audio story)"])