"You" Severide looked from his one friend to another. "You planned it from the very beginning!"
"You sound like you don't want to see me."
"Oh no, you are exactly what he wanted, blond and gorgeous." Shay laughed and everyone joined her except for Severide.
"Very funny" He growled still trying to sound offended but his friends' joy was too infectious. "I take it your strange behavior that day was part of the plan."
"Quite the opposite. Your reaction was the reason we came out with this plan." Shay admitted. "Just can't miss the opportunity."
"Of course, you can't." Severide muttered. Then he realized what she said.
"'We' came out with the plan? You mean you teamed up against me once again."
"Yes, Kelly." Casey decided to step in. "I've got the whole lecture from Shay about you being antisocial after my departure and how I shouldn't have left you for so long. That's why when she said that you became suspicious I suggested making this surprise-party. I thought it would be funny but now I'm here and you still look sullen."
Severide looked at his friend and he couldn't decide what he wanted more to hug him or to hit him. But what really matters was that Casey was back here with him and thus he had all the time to do all he wanted and to share everything he wanted to share with his friend during these last months.
"So, should I leave?" Casey asked prompting Severide to react and react he did.
"Oh no, Mister. You're not leaving, no way." Severide growled stepping forward with murderous expression on his face but in the end just hugging his prodigal friend. "It's been months. Where have you been? Was it so hard to let us now that you were okay?"
Shay and Clarisse stepped back simply enjoying the scene in front of them with Severide grumbling and Casey staying there with amused expression.
"Does he know how cute he is in this father mode?" Clarisse whispered.
"I don't think so. Luckily women still see him only as Casanova or he would already have few little ones on his hands." Shay smiled.
Few minutes later girls finally decided to step in.
"Okay, Kelly, if you want to get any answers you have to shut up and let Casey talk."
"I have to? And don't you think you had to tell me the minute you knew he was back? How long it's been by the way?" He looked at Shay and Casey accusingly.
"I've learned that day you caught me singing." Shay admitted. "Don't even ask!" She warned Clarisse and Casey.
"Honestly I think it was sheer luck from my part that I caught him. I'm not sure when he would announce that he was back otherwise." She glared at Casey.
"Now you decided to team up against me?" Casey arched his eyebrow. "At least you, Clarisse, are on my side."
"Oh, no, don't pull me into this. And by the way it's time for breast-feeding." With that she went upstairs.
"Now you're officially alone." Severide chuckled, the good mood returning to him fast.
"Okay." Casey lifted his hands in surrender. "I was going to arrange my return to the firehouse and all the paperwork before telling you anything. After all if Boden wasn't going to have me back…"
"As if that could ever happen." Shay snorted. "Always so pessimistic."
"Well, some things never change." Casey smiled at his friends aware that they couldn't stay angry at him.
"I guess, the fact that I was allowed to know means that you're back with us Lieutenant?" Severide asked.
"Yes, I'm back to protect you, squad boy."
"We'll see about that." Severide grumbled but relieve was obvious. "Now about you. How have you managed to make Otis lie to me?"
"It's all my irresistible charm, Kelly. One smile and he was ready to do everything, just like you." Shay winked at him flirtatiously.
A/N: Well, now you all know what the secret was about though I'm sure you guessed that already. The next chapter will be more about the time Casey spent alone, after all the gang is too curious to let it go easily.
Me, hope I managed to answer your question, there really was no particular reason for their actions.