"A tipped off B" means that A shared information with B that they would not be expected to know. "B was tipped off" means that B received information that they would not otherwise have. The sentence means "MK members suspected that" someone gave the authorities information about Mandela's location.

Hi Alyosha, how are you? Tipped off here means that important information was leaked (released) about Mandela's location. 🙌

given information discreetly

tipped off means" told about" in this context