I tend to feel Sonic stories ought to have a 'version of reality' explanation attached to them, there's so many variations of canon. This one refers fairly explicitly to Fleetway's and the earlier games' version of affairs in that the basic premise is that Knuckles is the last of the echidna and doesn't remember why... It does rather diverge from there...

It was in Ice Cap Knuckles had found the information. Deep in a cold, dry cave, below even the ice caverns. The paper - if it was paper, for it was slick and stiff and almost wax-like to the touch - was near perfectly preserved there in the dark and the cold and the dry, still air.

He spent a long time there, looking, his eyes and tongue dry and uncomfortable, limbs stiff from the chill and inactivity, head aching from squinting in the flickering torchlight. He moved slowly, thoughtfully, even after surfacing, until the snow and wind drove him into a run back to more clement areas of the Island, there to further study the rough sketched copy of what he had found.

It took him a weeks to decide to act on what he'd found. Weeks of indecision, whether to believe it, whether to follow it, whether to tell anyone. When he decided "yes" to all but the third, it took further weeks to act on it. Slowly, carefully, planning. There must be no dramatic changes of routine, nothing to tempt the curious, nothing to draw attention. The Island, and its Emerald must remain safe, distant, inaccessible, regardless of what he found or didn't find.

Summer on the surface would be best. He could glide a long way on summer thermals, cover further ground in the cooler evening hours, be a long way from the Island by the time he was likely to encounter anyone who might ask questions.

So he waited through the month and a half to early summer. Sonic arrived unannounced in a blur of activity and noise and Tails's chatter and an overwhelming impression that he'd popped in after just a few days instead of half a season. And then he was gone again and since his visits, while random, were generally infrequent, his departure presented a window of time.

Knuckles collected the pack he had prepared, walked to the edge of the Island and looked down at the western horizon where boggy grassland slowly gave way to thick, tangled jungle. The boundary stretched for as far as he could see, but that was okay. He didn't need to check the sketchmap at the top of the pack. The broadest river leading west across the open plain entered the jungle at a near perpendicular angle two thirds of the way towards where the view became misty with distance. It was a start.

He hesitated one last time, there was no easy way to change his mind past this point, but he had prepared, and really he had already decided.

He fixed his eyes on that river, checked the security of the straps on his supplies, and launched himself onto the warm air from the land below.