It certainly is possible to be discrete about murder. Clearly there are unsolved murder cases. The perpetrators in these cases are not included in data. Certainly, it is possible also for other murders to occur and for no one other than the perpetrator to know about it. e.g., 1) a person in poor health or an infant could be suffocated and it could be determined to be natural causes 2) a person could be poisoned, and this might not be identified 3) you could push someone off of a cliff and report it as they accidentally fell 4) a drug addict could be given a fatal overdose and it could be assumed to be self inflicted 5) a person could be murdered in such a way that it is believed to be a suicide. 6) etc. 7) etc.
I don’t want to derail this thread further by responding in detail to each of the examples above, but all except perhaps number 1, are basically impossible to get away with.
Unsolved murders don’t imply discreteness as much as they imply police ineptitude, apathy, or chance.
Then in the service of not de-railing, we can agree to disagree on the various possible ways of getting away with murder, my main point is that some perpetrators of homicide are not included in the data that we have on murder rates, thus our data may be suspect when it comes to establishing the rates of some classified group vs another.