"What happened to your chest?"

[Peep]

You could always just look down at your scars, gasp, and say "when did those get there?"

Double incision and inverted top surgery scarring does look remarkably similar to double lung transplant scars so you could say that. Or just that you don't like to discuss it.

"What scars? I can't see any scars?"

[FtMitch]

I guess I could say I was thrown from. A horse and landed on a cross country fence.  I have had enough horseback riding injuries that no one would question it, lol!

[2fish]

I had a dirt bike accident; I cut myself on the plastic windshield. Had to have my whole chest reconstructed. I was lucky; they were able to save my nipples. lmao 

[Tysilio]

...or better yet tel them they are gills.

I'd probably say something along the lines of having growths removed, but it sure would be tempting to say "I was born with gills."

[sigsi]

If you had to say something, I would go with having some growths removed. To make it more elaborate (and potentially shut people up if they try to get more info) you could say your *insert male relative* had something similar to it as well and act like it's no big deal / a genetic thing. Best I can think of as I'm not really planning on leaving the house without a shirt.
Good luck.

[blink]

I sidestep this issue altogether and keep my shirt on around others. But here's how I've handled other (not necessarily trans-related) questions in the past, that I didn't want to answer:

"That's a personal question."

Said matter-of-factly as you can manage. If you want to soften it, include a polite smile and slight furrowing of the eyebrows to indicate discomfort.

I have yet to have someone not drop it immediately. Yes, they're uncomfortable. But the moment they ask me a question for no good reason, which I don't want to answer, I have two choices. 1) Share information I'm uncomfortable sharing or 2) Let them be uncomfortable and keep my business to myself. I value my privacy more than their unbridled curiosity.

[sfreit89]

I generally say something sarcastic.. "Oh sword fight.. I won." Generally people take that as a hint that I don't want to talk about it and drop it. I thought about other conditions that may have similar scars but as someone mentioned above that seemed like a more elaborate lie and I didn't want to have to come up with answers to follow up questions.

[FreakOfNature]

I am in no way joking about this topic myself because I know that I personally will never take off my shirt in public due to my scars (unless I get tattoos maybe?) when I don't even want to see them myself but, I just literally thought about something like:...

"I used to cut myself due to depression and I missed the heart" or plainly "I used to cut myself"...

For a more genuine answer, I would say that I had an injury because that's how I see this kind of surgery with such an obvious evidence.