I had it on a table and it was still doing it. I am going to go to the gym tonight to do a sprint test. If I get my heart rate right up high then record it then send it to myself.
But I am having problems with mine anyway. I think my heart rate monitor is faulty. I keeps auto pausing during a workout.
No I mean the screen reads your pulse from your fingers (apparently).
This could totally be something apple store employees told people using the demo unit to avoid them asking how it's doing it without it being strapped on, I am skeptical as to why apple would put so much effort into recognising pulse through the screen for something so niche.
Your method of testing seems a logical way to see if this is accurate or not.