Mickey. In Traverse Town. Encountering a being of darkness/Nobody.
YouTube - Re:coded, Japanese cutscene: 04 - Closed Room
Fast forward to the 1:00...or don't, if you've already seen the Pluto scene where he runs into a Corridor of Darkness. Only beings of darkness and Nobodies can summon those, and neither Mickey nor Pluto are either of them. Therefore, someone else had to summon it, someone who was possibly related to Mickey's visit to Traverse Town.
I think that pretty much confirms a spot on BBSv2. As to what he was doing, well...After debugging Traverse Town, the King says to Sora, “That day, your travels began here. Also on that day, I was here too.” Does that mean that the King was in the same place at the same time as Sora adventured into Traverse Town in Kingdom Hearts I?
Nomura: Seems so. What he was actually doing becomes the previously mentioned “blank time period”, but in this title, it’s implying that the two crossed paths in the same place.
YouTube - Re:coded, Japanese cutscene: 04 - Closed Room
Fast forward to the 1:00...or don't, if you've already seen the Pluto scene where he runs into a Corridor of Darkness. Only beings of darkness and Nobodies can summon those, and neither Mickey nor Pluto are either of them. Therefore, someone else had to summon it, someone who was possibly related to Mickey's visit to Traverse Town.