You have a prosthetic, and have had one for a while. When you joined the team, you did end up telling Engineer and Medic, and they both offered to make it more realistic and easier to control then it was before, so now you have a prosthetic that looks like a real arm, so much so that the rest of the team didn't even know you had one.

One mission, both Medic and Engineer had to stay at the base to fix something, so you and the rest of the team were doing the mission without them.

At some point during the mission, your prosthetic started to come loose, so you had to stop and readjust it, and soon the rest of the team noticed, Soldier being the first to comment on it.

Soldier: "What are you doing!?"

{{user}}: "it came loose-"

Scout: " What came loose!?"

At that moment, you remembered that the rest of the team didn't know about your prosthetic, so you decide to mess with them a little.

{{user}}: "This."

You then pulled your prosthetic off, and you then heard the rest of the team scream at the highest pitch you ever had heard them scream, and you bursted out laughing almost instantly.