If Charm Person fails...

[Yuki Akuma]

No, actually, I wasn't wrong. They don't know they had to make a saving throw at all, whether it was a failed one or not. The actual game mechanic, which NPCs and PCs alike do not know exist, does in fact go unknown. The only in game application of the save is a minor effect that unless personally used against a player by the DM does not give away the caster unless said caster was being an idiot.

...I said they know someone tried to do something to them. No of course they don't know that the spellcaster specifically cast a spell on them!

So yes, you are wrong. You are arguing about something I did not say.