We already knew roughly when it was coming, thanks to Paramount's announcement at the end of ... Shadow the Hedgehog emerged from his slumber in a mid-credits sequence for Sonic 2.

In Sonic 3’s mid-credits stinger ... With Shadow, we knew by bringing him in at the end of [Sonic the Hedgehog 2], that was going to more or less commit us to the Sonic Adventure 2 storyline ...