[narfquat]

Perhaps corporate espionage/sabotage? Or maybe to keep things relatively low-key around the site.

Or perhaps the tech/approach is untested, and they want to do some preliminary tests/dry runs before putting their branding all over it.

These security measures are pretty tame in the grand scheme of things though. I think if google did want to completely mask their involvement, they would have at least issued different badges and credit cards to their employees on site.