It's a day of high intrigue at Facebook headquarters, with CFO Gideon Yu leaving abruptly. So why was former COO Owen Van Natta, who left last year, spotted walking out of a Facebook office?

There's an innocent explanation: His digital-music startup, Project Playlist, which recently scored a deal with EMI, has its offices in a downtown Palo Alto building shared with Facebook. But that doesn't explain why our tipster, who spotted Van Natta twice today, saw him walking from that building to another Facebook office. Could be a coincidence. Or it could be that Van Natta, who remains close to Facebook executives like Chamath Palihapitiya and Dan Rose, is up to something.