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Sonic Unleashed was released in late 2008, so 2004 seems like a very plausible date for an engine prototype.

Just because Hedgehog Engine was developed by (now non-existing) Sonic Team, it does not mean that they are the only studio that is "allowed" to use it. Mario and Sonic at the Olympic Games used it...

If they are now looking for these positions, then chances are that the game still has 2 to 4 years of active development before going live, depending on how much content they intend to "pre-produce"...