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I wonder how hard it would be to launch small payload to the moon without being noticed.Hard. The US (and probably other militaries) has a whole bunch of satellites monitoring the planet for ballistic missile launches, and so would detect any rocket launches.
After that, the US also a bunch of stuff monitoring space junk, and so any payload would be picked up on radar. It would not take many observations to calculate your payload's orbit, and if that orbit suddenly changed it would probably be noticed.
Now, if you suddenly change your payload's orbit to start a trajectory to the moon there would be nothing anyone could do about it, but it would at least be detected.