[pdkl95]

That would create very conspicuous network exit/entry points at the office. According to Zoz's DEFCON 22 talk[1] about modern OPSEC, these are specifically targeted by various agencies, so I'm sure Facebook finds entry/exit data points just as interesting.

[1] (warning: strong language) https://www.youtube.com/watch?v=J1q4Ir2J8P8#t=2291

[MOARDONGZPLZ]

Or, more interesting even. It is clearly someone taking the initiative to take their phone off the network at specific times in a specific place.