Gray Paper know Yesung enlisted in may 2013.
And you know his last solo is called "Gray Paper", and he sang it at the only SS5 he could participate in.
And you probably know Kyuhyun said on Twitter that his solo stage was "really really good".

Some months later, something happened in SS5 Tokyo (130727) : Kyuhyun and Ryeowook sang Gray Paper (fancam)
And after the song...

Ryeowook: "Kyuhyun was the one who came up with this idea of performing Gray paper"
Hyukjae: "So Yesung didn't know about this?"
Kyuhyun: "I only told him there would be a surprise for him"
cr: siohappy

Kyuhyun: we were thankful about KRY receiving lots of love in Japan, so we prepared an event (to make the stage look like we are singing w/ Yesung)
Ryeowook: Kyuhyun-ssi planned this stage
Eunhyuk: But the important part is that Yesung-ssi doesn't know about this, right?
cr: NKsubs

KH: I'm not really close with Yesung-ssi...
RW: I am close with Yesung-ssi!
EH: We could have done the stage with me and without Yesung-ssi, so why..
RW: Kyuhyun-ssi prepared the idea
cr: NKsubs singing Gray Paper was Kyuhyun's idea.

Yesung posted the link of the fancam on Twitter saying :

俺を泣かせるなんて! ありがとうございます。^^
It made me cry ! Thank you ^^

Fancams of the second day (130728) : here and here ; members talking about Yesung here 

Sungmin : Gray Paper was Kyuhyun's idea!
Kyuhyun : It was secret for Yesung. Then I talked to Yesung last night, he just said one word, it made me cry.
cr: wwmisa

Kyuhyun was doing the 'dont mention it' action when Wook said the project was his idea 
cr: morningsuju

And then, some fanaccs of fans at Mobit :

My friend tell Yesung that Kyuhyun was tearing at last night’s SS5 Tokyo… He replied “Ah~ really? its too hard to see Kyu tearing”
cr: kklee1995

Ryeowook or Kyuhyun? Please choose one of them…
Yesung replied “I can’t choose… they are both my lovely brothers…”
cr: kklee1995

Did you know that Gray Paper was Kyuhyun's idea ?
Yesung replied “Ah~ yeah, I don’t even know that Kyuhyun will be sweet and care about me…”
cr: kklee1995

(YSharon_ : The following is the fanacc by my two friends who went to Mobit today. They are not Elf though.)
Fan A: Hi, Oppa!
JW: Hello (said gently)
Fan A: Do you mind if I ask you several strange questions?
JW: Strange? I don't think so... (shake head)
Fan A: (asking them anyway) Who do you like in EXO?
JW: (Said immediately while shaking his head) I don't know.
Fan A: Oppa can you choose one, please!
JW: Is this that important (to you)? (after 5 seconds later) D.O!
Fan B: Oppa oppa! Then between Kyuhyun and D.O who do like better?
JW: They are both my lovely dongsaengs. (said with a straight face)
Fan B: Oppa! Please choose one! I had ordered 10 drinks today!
JW: Um, I like Kyuhyun!
cr: YSharon_ & siohappy

SS5 Taiwan (130810-11-12) : Kyuhyun and Ryeowook sang Gray Paper again (day 1, day 2)

Kyuhyun: Do you miss Yesung?
Fans: Yes!
Kyuhyun: Do you know Yesung?
Fans: Yes!
Kyuhyun: Who is he?? (And then curls his fingers to imitate Yesung's small hands)
cr: kikiikyu

--> fancam here

And it has been said that Gray Paper will be performed at all the future SS5 !

Siwon: Do you guys miss Yesung?
Fans: yes!
Wook: The stage we had just now that performed with Yesung was actually Kyuhyun's idea.
Hae: How did you come up with this idea?
Kyu then got all shy and used his hand to make a gesture like 'don't talk about this anymore'.
Hyuk then said '你不是不喜欢艺声吗(Didn't you say you dislike Yesung)?'
(Kyuhyun then went over to stop Hyuk from saying more, and then return to his spot)
Kyu: actually Yesung hyung really wants to be here on stage with us. So, want to sing together with Yesung hyung on stage for everyone. What do you guys think?
(fans clapped)
cr: siohappy

--> fancam here (1:30)

Kyuhyun wanting to stop Eunhyuk from saying more

Official Video