If you search for Y Combinator on Google Maps it gives you a giant circle centred on Mountain View that spans all the way from Palo Alto to Santa Clara. Address is definitely Mountain View, though.
Finally looked at it and it's because the address is generically "Mountain View, CA" in Google Places. I tried to suggest an edit to give it a more precise address but it wasn't one of the available options. Perhaps YC asked for their address to not be listed so random people don't show up to pitch.
Interesting, I've never seen that before.