I couldn't find the answer to this question in Kanon 2k2 or the 2k6 version. I have not played the game nor will I plan to, but since KyoAni left it out in the 2k6 (or will it be explained later? I hope it will), I'm guessing it's safe to assume it was not explained in the game, also?
Ok, we all should know Yuuichi's
purpose for coming back to his childhood city, by now, to recover his lost memory and fix the events in his past. What we don't know is what made him return there. Did his parents forced him? Why did he suddenly decided to return to the town he has very little memory of? In the begining of the first episode (for Kanon 2k2 and 2k6), there was no explaination on why he just suddenly show up in a town he used to visit during his childhood. For Kanon 2k6: It was a quick introduction of him and his cousin, then move right on to the next day, the next morning waking up at his aunt's place...
Well if it isn't even explained in the game, I guess we'll never know the answer? IMO, if that's the case, that's one HUGE flaw to the story telling. Yuuichi will remain more mysterious than all the other characters, combined!
Oh and I found my answer about whom Shiroi was walking with in that scene, after re-watching "Kanon: Kazahana" again.