Smartwatches sense hand activity

True. Although given the effort going into beacons, always on bluetooth (LE) and tracking other bluetooth and wifi clients phones are getting pretty good at telling what other devices (and their owners) are in close physical promixity. Apparently it's not uncommon for phones to be able to make sounds outside of the range of normal hearing to use the microphone to detect other phones in proximity.

So not only might a watch figure out who is wanking, but also who else is in their immediate proximity.

That doesn't work, because next time instead of the watch it will be some other "smart" device (maybe a heating system that measures your body temperature), and you won't have read the article describing the problem.

Took watch off for N minutes during the day? Detected: probably wanking.

If that one lady did that a few years ago, around my area, she wouldn’t have been convicted of murdering he husband. The Fitbit is how they caught her.

Perhaps google/apple, from looking at the traffic. But the random app from a close-to-anonymous publisher don't see the traffic (unless they VPN your device), but they may see the accelerometer data.

If they're not doing this already, they'll be doing it in <5 years. There's currently no downside to dragnet data collection of anything and everything you can get your hands on.

Yes.

True definition of ‘hacker’.

Now I'm not sure if your username really is about shells...