Not just a Nigerian problem...
There was a Learjet that was missing for a long time in New Hampshire, U.S.A. It had no ELT, since there was a loophole in the rules that meant jets didn't need them as small aircraft did, and even though it must have crashed shortly before landing it took a very long time before a hunter finally came across the wreckage.
Even in a heavily-populated country such as the U.S.A. you can still get large areas where there are very few people to notice a crash. Too, when you do the maths for an aircraft that is doing something like 250 knots for even ten minutes then you end up with an impossibly large search area. In that case it would mean searching an area of about 1 400 square miles!
As we have all been taught, the approach and landing phase of flight is one of the riskiest, with CFIT always a possibility. Obudu Airstrip has no approach facilities, although I suppose one could make up some sort of GPS-based approach, plus it is in very hilly terrain, so that it would be easy to imagine a small mistake having deadly consequences trying to land there if the weather is not CAVOK.
Having total radar coverage (as in the U.S.A.) is no panacea to this problem of aircraft going missing. You usually find that once an aircraft descends below a certain altitude then it goes off the radar, as this unfortunate Learjet did, when you are reduced to guessing just where it may have crashed. On the other hand, if the crew are either hijacked or else want to "do a runner" then there really isn't much use to having radar coverage. Assuming ATC are even watching closely there are rather simple ways to disappear from the screen we can all imagine.
When I was working down in south Florida we had the opposite problem, aircraft flying into the States smuggling drugs trying to evade being spotted on radar. It took some complicated solutions, believe me, things that would be far beyond the capabilities of normal ATC radar coverage.
If this missing 1900 really is the subject of an investigation I think you shall find there are legal devices that mean it easily could be used as evidence without having to have the aircraft physically present in court. To say that the aircraft's disappearance means it no longer comes into question as evidence sounds like a red herring to me.