So I actually received one of these notices from Google.
A few years ago detective in Massachusetts had subpoenaed all information having to do with my GMail address.
Not having done anything wrong it was very stressful time for me trying to figure out what the police were after.
Turns out some scammer had put down my email address (a common first name/ last name combo) on a form at a UPS store.
UPS being the terrible company it is didn't bother to verify said address. (And BTW refused to remove my address even after I repeatedly called their help line).
In the end I had to hire a lawyer a thousand miles away in order to contact the detective to try and clear up the situation.
Five things stick out in my memories of this incident:
1. If I was so inclined it would have been really easy to destroy all of my computers given the advance warning.
2. They attack surface was bigger than I could imagine. Given that I payed for Google Drive and used Google DNS Google knows everything about my online life including all the illegal things (copyright related) that the fuzz might find despite looking for something completely unrelated.
3. It is super easy to find oneself in a kafkaesque type situation where one if forced to try and prove a negative. "Yes that is my email address but I swear I never scammed someone out of their iPhone. Yes I know that's what actual scammers would say."
4. It would have been super easy to dismiss the warning email as spam.
5. They don't tell you why you where subpoenaed. I get Google doesn't know but it does leave (innocent) people to spin out into a paranoid frenzy.