Hey there! Sorry for the delay getting back to this -- he will discover that the department store was not built on any notably cursed or dangerous location, and in fact it looks like the proper purification ceremony was performed before they broke ground for the construction (there's a nice little newspaper article about it from a few years ago.)

If someone catches the koi, they will behave like normal fish unless/until released, at which point that person may receive a reaction ranging from having a fish make a rude "thbbbbt!" noise at them, to being thanked in perfectly understandable, if slightly water-garbled, Japanese. Their response will depend on how prompt and polite the angler was about releasing them.