People across the internet are calling this "a hardware Easter egg", but it seems more random than anything else. If you buy a Google Pixel or Pixel XL then take out the SIM tray and look very closely, you'll see... Wait, what is that?
It's Google's address. To be specific, it's the address of Google's headquarters, though in slightly shortened form. The full thing is 1600 Amphitheatre Parkway, Mountain View, California, 94043, the United States of America.
Why would you want to see Google's address every time you put a new SIM card into your Pixel? That's a good question, isn't it? And if you have the black model of either phone, the text is very readable and does stand out (it's less so with the silver versions). Could this be a semi-stealthy (or at least harder to find) nod to Apple's famous "Designed by Apple in California" inscription? Maybe. No one knows for sure - perhaps not even Google. But there you have it. Your Pixel comes with an apt reminder of the exact place on this Earth where its maker's HQ is located. See, you do learn something new every day.