Ive not been shirtless yet (well, in the country I live in. I did once when on holiday with my girlfriend. I thought Id make my first time somewhere noone knew me as I knew Id be very self conscious!)
But I had to take time off work for surgery, and my friends at uni knew I would be away a couple days for surgery,so I had to make an excuse.
I briefly explained that I had a problem with the muscles in my chest, around the pectoral area, and it gets tight etc so I have to have an op to help with it etc.
Funnily enough, atleast 3 people I told, before Id finished my made up explanation, they said something along the lines of "Oh, I had a friend who had something similar to that. Is it (bla bla bla)?". I was just like yeah, something like that. (Thanks for filling in what I needed to say :p ).
I found telling my friends this, it helps me for if/when they do see me shirtless in the future, they already know I have the scars and what its from etc, so its no surprise.