I'm kind of on the fence on this aspect. I've worked in some pretty big multi-occupancy business buildings - if people come around wearing high-vis and acting official no one blinks an eyelid - dress it up as a HVAC maintenance or whatever and you can move a lot of equipment around and get access to all sorts without people giving it a second thought (obviously would be remembered afterwards). Unless you were hanging explosives and miles of detonating cordage off support structures, etc. people aren't going to really notice. With conventional demolition though you'd probably have to use some pretty big charges on unused floors to make up for not being able to stack supports at will and that would be very noticeable going off - not just "maybe it was demolition charges" it would be massive flashes and distinct booms of primary charges before any collapse (IIRC there is usually a sequence of charges as well to bring a building that big down). On the other hand from the days I was working in R&D even back then there was tech i.e. specialised shaped charges either functional or in development that could work around the conventional aspect so while unlikely in that respect I can't rule it out. The harder bit is pulling the whole lot off without anyone ever talking down the line given the number of people it would have to involve even assuming that was via a smaller number of actors controlling the actions of a larger unwitting number - most people don't question someone superior to them when their attention or efforts are directed, etc.