I've seen how the first floor looks nowadays. But if you weren't ironic with your reply I'd take in teams with IR detectors to scan the perimeter walls and pavements, starting from where Brian left behind the office spaces behind by the service elevator and down to the map out the entire ground floor on street level, to look for human signature. Those gadgets can be hired even nowadays if not purchased. They draw waves on a screen and shades depending on the material behind in both dense material and void spaces up to a certain range behind the sensor. I'd then move to request blue prints for the foundation of the building and check with expertise the layout of pipes and sewage system, and possible void- and crawl spaces thereof. I'd tried to get beneath the building if possible through sewers and technical entrances and sweep those area, and then again with the same detectors. And Investigate the historic tunnel system thats was uncovered at one point. I'd of course try to contact the former project manager for the complex to try to interview whomever was responsible for ongoing operation in construction. I'd also re-check whatever was brought in and out of that building at the time and make inquiries in this regard. There probably tons of stuff in those case files that can be re-investigated concerning the Gateway building at the time Brian went missing. If there's a will, there's a way.