It's not actually proximity based. Really, keeper is a terrible solution.
The handset has a reliable sensor for detecting if the SPen is speed in the phone. If it is not, the keeper app uses the phones internal sensor to detect if you are walking away from the pen. This is probably detection of vertical movement or something of the sort. If you're light on your feet it won't work.
There are slews of reasons it fails to detect you moving from the pen. I think they added it just to have it on the feature list. This is also probably too refute a customer saying they'll probably lose the pen to encourage them to buy the phone.
Let's also remember to put a feature in the pen like a sensor requires power. Having a customer buying batteries for their stylus would be terrible, so they simply did it this way. Sometimes giving a good user experience, gives a bad user experience. If that makes sense.
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