Q:

How does Mordred know that Merlin is Emrys?

In the end of episode 5x11 "The Drawing of the Dark", Mordred reveals Morgana that Emrys, whom she has been searching for long, lives in Camelot and is, in fact, our hero Merlin.
My question is: how does he know? Some thoughts:

The only people who know Merlin's identity are Gaius, Alator (who has known that from Gaius) and his disciple Finna (and Kilgharrah, but I don't think it matters here);
Alator refused to reveal Emrys' identity to Morgana, at the cost of his life;
Finna, when captured by Morgana (end of 5x10 "The Kindness of Strangers") kills herself before telling anything to Morgana;
Mordred knows that Merlin is a Sorcerer but, as he says while confronting Morgana in 5x09 "With All My Heart",

MORGANA: [...] Where's Emrys?
MORDRED: Emrys?
MORGANA: You pretend you do not know of whom I speak?
MORDRED: It is a name I've only heard of.  

So my question is: how can Mordred, in the end of the episode, know Emrys' true name? When did he became aware of that?

A:

Mordred has known all along, going back to his very first appearance in season 1; in that episode:

He comes to Camelot and meets Merlin
He realizes that Merlin is "Emrys", a figure in the lore of his people (the druids):

Mordred* [telepathically] Thank you, Emrys.
Merlin: [telepathically] Emrys? Why do you call me that?
Mordred [telepathically] Among my people, that is your name.
Merlin Season 1 Episode 8: "The Beginning of the End"

Quite how he recognizes Merlin is never made entirely clear, but Kilgharrah later hints at some secret lore:

Merlin: How does the boy know who I am? I've never even met any Druids.
Kilgharrah: There is much written about you that you have yet to read.
Merlin Season 1 Episode 8: "The Beginning of the End"

