The number of Apple people on Mary is tiny; maybe a few hundred. Infinite Loop and surrounding has many thousands. Over the years, Apple has leased many additional buildings surrounding Infinite Loop. Campus 2 will have another 6-8000 or so when done. Apple may very well move out of some leased buildings at that time.Actually Apple did move to North Marry (part of IS&T) moved from infinity loop.
It has to be some huge company 1 million sq.ft is really big. I don't know but thing is that when Apple 'expanded' or moved to marry half employees did know for that couple months in advance. I think that would be also case with this big movement. It has to be some other company.
What about SAP? They converted HP Pavilion to SAP Center.
No one will be moving into the Brokaw buildings for a couple of years. If it is Dell, I suppose that manufacturing will stay in Texas and administration, design and new technologies will move to SJ. I doubt that people have been informed, but management must have a pretty good idea by now that someone is moving. The people I know at Dell say they don't know, which is what I would have expected.
SAP is certainly possible. But it seems unlikely to move that many people to the Valley since typically only top level design people are in the US (the rest are in Asia, etc., where salaries are lower).
Amazon is also possible, although the rumor is that the new company is a company with a HQ in the Valley but nothing in SJ. I'm not sure what the several thousand employees in SJ would do if it is not a HQ (and I doubt that Amazon will move it's HQ out of Seattle).