Definately scary, and if you're like me you want to not worry unecessarily, but at the same time check it out thoroughly to make sure it's really OK.
My son had something very similar on his sternum over a couple of years ago. There was (and still is) a lump, like a raised asyemmetrical protrusion just on one side of his breastplate. I took him to his dr. who referred us for a chest x ray. Turned out they said nothing serious, but was likely a over calified growth possibly from an injury. Like the body's "over healing".
So I worried more thinking what horrible thing did I do to allow him to get injured. About 6 mos. prior he fell off a golf cart (long other story, with an older cousin, dad supervising) He had the wind knocked out of him but no other signs/symptoms of anything else. We didn't even take him to the ER. Dr. said that this may likely been the hit that caused the "over healing" but nothing to worry about. And that even if I had taken him to the ER they wouldn've found anything. (I still feel bad, though).
But, I still watch it and he's almost 6 now. It happened when he was 4.