I have both an Apple watch and an Apple phone that share data, including their locations, when Bluetooth is turned on. My devices aren’t connected to the cloud or iTunes so no data is externally uploaded or stored. You would need my watch to find my phone and vice versa. My watch can also continue to function for around 30 minutes in not too deep water.
If Samantha’s data (health and location) was able to be accessed early on, either she had the cloud or iTunes turned on, or her watch was found. Through the cloud or her watch they may have been able to discover information about her activity and even the location of her phone. Maybe her family assisted accessing GPS data through Google.
The photo here is from my Apple watch with the “Backtrack” function able to record someone’s route. Also of course, the watch can detect falls and call emergency services.
Where is her pink watch? It’s visible in the only photo (which was also cropped) released of Samantha on the morning of her disappearance. A replica was on the table in the UI show which aired just prior to the arrest, and I thought it’s placing and presence was intentional.