I imagine that Google is keeping a log of the ip addresses that phone reports when it's connected to wifi. I There's probably some overlay between you and your known associates / locations.

They also log all the locations where anybody's phone encountered an SSID.

It's unclear whether an SSID suffixed with _nomap truly opts out of the collection, or whether it simply opts out of later sharing.

Even if they don't, the law enforcement definitely does (or rather requires your ISP to).

So depending on who you are trying to protect against you are in for varying amounts of fun.