People keep disappearing them reappearing again. But the ones that appear aren't the same as the ones that disappear. Sounds like a job for Boffin Boy
Of course it's a door into another world and Boffin Boy and Ninja wizard Wu Pee set off to find it. When they do then the story really starts...
Buy Boffin Boy and the Lost City by David Orme from Australia's Online Independent Bookstore, BooksDirect.