It is a longstanding prank at Microsoft's main Redmond campus to send an unsuspecting new employee to building 7 under the pretense of having a meeting or needing to pick up something. There is no such building on the main Microsoft campus. This nonexistent building has also been used as a sort of inside joke. For example, if somebody invites you to a meeting in Building 7, they're probably inviting you off campus to take a break from work.
A few years ago, there was a vacancy in a small business park a very short walk from the main Microsoft campus. (You don't even have to cross the street to get there.) I thought it would be great if somebody leased the space and opened a bar called Building 7. It could have a back room decorated like a Microsoft meeting room, complete with WiFi and a large monitor. Then somebody could legitimately invite you to a meeting in Building 7, and people could use it as a cover story: "Sorry, I'm going to be working late tonight. Team meeting in Building 7."
The announcement in February 2006 of a major expansion of the main Redmond campus came with a little map, and people with way too much time on their hands pored over the unreadably tiny print on the map and noticed a little 7 printed on the orange building in the upper right corner. They quietly fretted that a piece of Microsoft history was vanishing and that they'll have to come up with a new prank to play on the new-hires.
In June 2007, pranksters (and people with a nostalgic streak) breathed a sigh of relief. The real estate department announced that the new building tentatively labelled 7 on the map will now be known as Building 37. In deference to Microsoft tradition, the name Building 7 has been officially retired. There will never be a Building 7.