Depending on how the calendering system reports busy-ness, there's a nice timing attack on figuring out who people are meeting with. If the calendar shows busy

Depending on how the calendering system reports busy-ness, there's a nice timing attack on figuring out who people are meeting with. If the calendar shows busy when an invitation has been sent, instead of when someone accepts the invitation, and you take snapshots frequently enough, you can probably group people into meetings based on when the meeting showed up on peers' calendars.