There is so much stuff packed inside the space station and there's such a high likelihood of things getting lost (you can't just set something down on a table and expect it to be there when you come back) that it's not surprising that things like this show up occasionally. This may have been on a lost items list for long enough that it was assumed to have been trashed at some point. Or perhaps it was still on a lost items list. Every item is tracked, and the inventory managers do a great job tracking it, but some items slip through the cracks... particularly items from 20 years ago.