Your location data. Tower associations can still happen with data "off", since there's plenty of "listen" components. All your home wifi connections are well Geo-located, thanks to other Android users picking up the ESSID as they walk / ride / drive past your house. Your shopping / outings? Forget it, fully known.
VPNs hide the content of connections, at least from MITM / eavesdroppers, but server-side data scrapes are quite effective at figuring out who you are (or what your phone is ... see below). Nothing really does a good job of hiding the fact that you are connected to a VPN except TOR, and where that connection originates (e.g., your wifi network, which is well Geo-located, remember?). And de-anonymization of VPN connections to identify downstream connections are possible, IIRC. Details about your phone are well recorded (MAC, SID, etc)
And always remember, your phone can be implicated based on location data, which will implicate you once it's discovered you own the phone. And that's as simple as looking up the SIM purchase / use.
VPNs hide the content of connections, at least from MITM / eavesdroppers, but server-side data scrapes are quite effective at figuring out who you are (or what your phone is ... see below). Nothing really does a good job of hiding the fact that you are connected to a VPN except TOR, and where that connection originates (e.g., your wifi network, which is well Geo-located, remember?). And de-anonymization of VPN connections to identify downstream connections are possible, IIRC. Details about your phone are well recorded (MAC, SID, etc)
And always remember, your phone can be implicated based on location data, which will implicate you once it's discovered you own the phone. And that's as simple as looking up the SIM purchase / use.