Having flown down the West coast of Africa fairly often in small aircraft, I think we drop out of Dakar radar about Bissau, and then get picked up by Abidjan at about Roberts. Of course, there is no way to be sure that we are under radar coverage, but this is the impression I have from the messages from ATC.
So no surprise that most of the Latin American drug planes come into Sierra Leone or Guinea-Conakry. I have no idea how they leave Latin America and get across the Atlantic without detection, but they appear to be able to do so.