We can assume that at the time of the accident there was other traffic in the area going northbound. The three parallel South Atlantic ATS routes are not heavily but frequently flown. Any distress calls on 121.5 would have been certainly heard by someone else. VHF is extended range up to 400 or 500 NM from the receivers in the area and Recife and Sal can listen far out. Also other traffic would have experienced the CB area yesterday night on the same airway, most likely also at the same flight level 350.