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
Edit 2: Double rings and the number 7 found in
another tweet:
Edit 3:
Sega blog posted about Hedgehog Day with some relevant info: