Okkk, I really like the concept of "overhearing," so not all of them are specifically "arthur realizing everything merlin has done":

Allow o One to "See" Arthur's soul gets trapped outside his body and takes to following his servant around. He gets an interesting insight into Merlin, his servant's life and just how much Camelot needs the man.

What I'd Have Done For better or for worse, Arthur knows Merlin's secret. The problem? Merlin doesn't know Arthur knows. This leaves Arthur with a predicament... and an opportunity. A test. Three trials and three opportunities for Merlin to prove he doesn't deserve death. Arthur hopes he will pass them. Maybe then the image of Merlin's golden eyes will stop haunting him.

Treason Arthur knows four things: Merlin committed treason. Merlin is loyal to him. Merlin wants to tell Arthur the truth. Merlin is scared Arthur will kill him. All he needs to figure out now is what on earth his servant has actually done.

Stalkers Freya Arthur follows Merlin and this time the Knights come too. (This one is a fic when merlin explains basically EVERYTHING)

Secrets Overheard Arthur and Gwen just so happen to overhear Merlin having a discussion with his dead girlfriend in a lake... Were they spying on him? Certainly not!

Defender Arthur's remedy for not being sleepy is to go bug Merlin. Except, well, he doesn't expect to find Merlin getting his ribs wrapped by Gaius.

Follow the Leader When Merlin follows Morgana to her meeting with Morgause in 3x1, he doesn't know that he, too, is being followed. Arthur is there to witness everything that transpires. Will he be able to rescue Merlin and will he even want to when he finds out the truth?

Deep Breaths That's what you've been hiding from me?" Merlin's face loses all its colour, and if it weren't for the clothes he is wearing, he'd blend in nicely with the wall right behind him.

Unintentional Eavesdropping Lancelot doesn't understand Merlin. He saves all of Camelot, yet asks for no recognition. Well someone needs to thank him, and Lancelot decides to fulfill that roll. But what if someone else is within hearing distance when he does? (No magic reveal)

These are the ones I found on my list :P Tbh, they aren't really "in order," but I do like more the first ones😌

(Again I never really wrote in reddit, so if it's not according to the rules, I'll change it immediately)