Whatever these symbols mean, they seem pretty in-your-face while also being semi-hidden. They appear to be the numbers 3, 21, and 00. Or perhaps those zeros are merely circles with dots in the center? Star rings, maybe? I'm not sure. In addition, Mike Pollock tweeted:
Which is silver, by the way.
Tumblr post, with higher res video
lock if it's no use or old
Edit: The Sonic account also posted this picture, containing some of the same numbers
3 / 21 were numbers that appeared in the Game Grumps photo as well.
Edit 2: Double rings and the number 7 found in another tweet:
hold up hold up hold up:
there's a tweet with this:
Could be wrong, but the seven and the double rings appear to be added from the original source.
Sega blog posted about Hedgehog Day with some relevant info:
I know at least half of you reading this are thinking: “Hey! Are you announcing anything big today, like Sonic Labyrinth 2 or Tails Sky Patrol IV? What’s the scoop?”, but you’ll need to be patient for a while longer. Good things take time, and we’re sticking to that. That said, you’ve probably noticed a few numbers floating around here and there. Who knows that they might mean? (By the way, in case you missed it – those weren’t eyes, they were zeros.)