Seems relevant:

A search warrant filed in California regarding a criminal theft case details how push notifications demands can be used to obtain information about a person. The search warrant, seen by TechCrunch, includes a section where an FBI special agent writes that when a user installs and downloads an app, the app directs their phone to obtain a push token, which is a unique identifier that allows Google to locate which device the app is installed on.

“After the applicable push notification service (e.g., Apple Push Notifications (APN) or Google Cloud Messaging) sends a Push Token to the device, the Token is then sent to the application, which in turn sends the Push Token to the application’s server/provider,” the record reads. Then, whenever a company sends a push notifications to a person’s device, it also sends Push Tokens.