How do you interfere with folks who had been long relocated to Europe?
I don't think it's that difficult. But before we get to that: Some witnesses were initially relocated to African countries and then relocated again or either stayed there. Yet others actually stayed in Kenya! In either case, I don't imagine that it would be too difficult to find them.
To the main point: Witnesses in "witness protection" program never actually lives of complete isolation from the past. I once looked at some data on witnesses in the USA system who had been found (and dealt with) by murderous former associates, and in almost all cases it was the witnesses own actions that lead to the "discovery"---a man missing a lady who used to give him a sweet something, or getting in touch with friends, etc.
Now, consider the African case, and the strength (or at least seriousness) of family relationships, all the way to "extended" families. At best the witness-protection program will relocate a man (woman), spouse, and children---not brothers, sisters, cousins, etc. Can we really expect a person in Europe to not reveal to brothers, sisters, etc. information on his whereabouts? And can we really expect that when it's the "authorities" (i.e. the big people) in Kenya who are interested that we can rule out the squeezing or offering of inducements to the relatives as a way to get to the witness? As I see it getting to an African witness, even if in Europe, is not especially difficult. More so for those with the power and money, but even so otherwise: simply start with the relatives.
Another thing to keep in mind is that this is Kenya, Land of Eaters, and who would miss an opportunity to take a bite? Take, for example Witness 800 (see Decision). He admitted, to the court, under oath, that he ate and also worked on (or at least agreed to help work on) other witnesses. That he seemingly did so without any coercion and in fact apparently lied to start with, so that he could eat to recant, does not absolve the bribers. More importantly, it puts paid to any claims that there was no witness-interference, and it certainly simplifies the case for witness corruption.
We'll have to wait and see ... My own guess is that money has been eaten in the Kenyan judiciary (Court of Appeal) to sit on cases like Barasa's. Whether that will be maintained remains to be seen, but I think we can count on the OTP not forgetting them; in their shoes, I would now appear "voluntarily".