How is a 3000-person game of hide-and-seek supposed to work? Hide-and-seek typically involves knowing who you're looking for, or at least knowing that anyone in a given area is in on the game.
I would probably organize it like a big game of infection tag. You start with one or a few people who are tagged, but each person who's found becomes tagged as well, and becomes a searcher. When you find somebody, you scan a QR code on their phone (or on a race bib that everybody pins to their clothes). A trivial database on your phone tells you who was found last, how many are left, etc.
I was wondering the exact same thing. Who hides and who seeks? I'm guessing that it's more like people go in small groups and do it within their own group but just with the size of the Ikea store being the fun factor. Or maybe they team up against another group of equal size to see which group can find each other faster?
Or maybe they wear some sort of tags in their clothing to identify them as hiders.