The Charm Person spell description says:
When the spell ends, the creature knows it was charmed by you.
There is an ambiguity here — does this happen only if the target failed on the saving throw, or the result of the saving throw is irrelevant?
For the sake of simplicity, let's say the target didn't see the spellcasting act itself. According to the rules, it still "knows it was charmed". Does this happen if the target succeed on the saving trow?
On the one hand, the spell ends, so the target should "know it was charmed". On the other hand, one might argue the spell never took effect, so literally nothing happens when the saving throw is succeeded.