Want to trade??? No matter how big my pecs are, there's a big dent below my shoulder blades. I can't fill that in for anything.
To answer your question, I'd say it's a lateral piece of your front delts. Those can get huge fast. You might be able to isolate it and figure out what it is, but flexing you front delt; maybe arm straight out to your side and a little high, and push on a door frame like you're hitting a tennis ball. That should pop out your whole front delts, without affecting the neck muscles. Other stabilizing or support muscles for the neck, probably aren't going to get big enough to notice, and they're tucked in behind the front delts and upper pecs.
Here's a diagram on another site. See how the front delts actually go way into the middle of the chest just below the shoulder blade, and disappear under the upper pecs. What a lot of people might think is upper pecs, is actually delt mass. Mine are a little different shape from the drawing, and what my problem is (causing the caved-in dent), is that my front delts are huge on my shoulders, and my pecs are a lot bigger at the bottom. No matter how I train those muscle groups, the outer heads of my front delts, and the lower pecs, get bigger faster. My avatar pic kind of shows what I'm talking about with the outer delts.
http://www.criticalbench.com/muscles/muscular-anatomy-front.jpg
Charles