The weirdest one for me was Google Timelines. I've had an android phone since 2011 and didn't find out about it until maybe 2 years ago. If you have your location services on Google automatically creates a timeline of everywhere you have been. Little creepy to go back and see what I was doing on XYZ date almost a decade ago now.
There is no phone app preinstalled for it or really any obvious notification Google is doing this in the background. I believe the only way you can open your timeline is on a desktop version of google maps and finding it through the sidebar.
Have they made a reality cheating show out of this yet? lol
Seems like a great and cheap way to catch ppl cheating and cashing in on it, especially since it's pretty trivial to take someone's phone, plug it into a computer, break the lock, and TA DA!