Tony Stark's mini arc-reactor in his chest that always glows. However, there are several scenes where the glow is not visible through the shirt he's wearing. I noticed this first when Nick Fury is talking to Stark & Captain America after the big fight onboard the heli-carrier. There is absolutely no glow visible through Stark's t-shirt. However, a few moments later, when Stark & Cap are talking, the glow is clearly visible through the exact same shirt.