ELT signals are picked-up by passing Satellites and many aircrafts listen to 121.5 newer models (EPIRB type) will give more info (Coordinates and registry info such as owner etc...). No ELT means: A non functioning or non installed unit. Exhausted or deflective Batt. Unit Destroyed unit upon impact fire or water immersion etc... OR... Someone turned it off... Conspiracy theories anyone ?
Kidnapping, UFOS ...