No one expects Ancient Egyptian Hieroglyphs!
Whether Ellen should be announcing this to the party is debatable, but I think it would be awkward keeping something like this limited to Nanase the player. The rest of the party would be aware she knew something in any case just by virtue of Ellen having to pass a note or something.
I think the implied mutual agreement of "my character isn't aware of everything I am, and I will roleplay as such" should fairly cover this.
Though I'm sure I'll hear opinions to the contrary if any readers have them, which will be interesting if it happens.