This is the ultimate sample bias. Organizations are perfectly capable of keeping secrets, even if they are composed of individuals evolved to communicate the truth. We don't know how much we'll never know.

If we find out about clandestine activities (by governments or by private citizens), it is often decades later. Given that the capabilities have existed and been widely discussed for some time, it doesn't seem any more speculative to assert that there have probably been assassinations as that there haven't been. Radio connections don't leave behind much evidence.