Upon assimilating a starship it is not unusual for the Borg to construct an interplexing beacon to establish contact with the collective. The Borg have done so on the USS Monitor in 2377 and the USS Enterprise-E in 2063 when Borg from the 24th century attempted to establish contact with the collective of that era. (TNG video game: Away Team and TNG movie: Star Trek: First Contact) Given the recent point of divergence, it doesn't seem too improbable that temporal interplexing beacons exist in the regular timeline as well.