Thanks for posting this! My watch was doing one of these few-second spikes once a week or so. I figured it was the watch malfunctioning, but you do wonder. But I figured it out.
I started paying attention to what I was doing when it happened, and I realized it was when I was doing my hair. I have really long, curly hair, and sometimes I finger curl it. Imagine wrapping a really long piece of string around your index finger of the hand that you wear the watch on. When you do that, your wrist makes a tiny, quick flicking motion as you wrap. I started turning on the heart rate app while I was finger curling my hair, and it's reproducible - I can make my heart rate spike to levels that are not happening.
I had read that the HR might not be accurate when you are doing vigorous exercise with a lot of wrist motion, but I was thinking like burpees or other big motions. This is just very tiny little motions. Larger motions, it seems able to handle. I think it's actually OK when I'm exercising - if my HR goes up to 170 then, I'm usually earning it. I think I've read that it uses a different sensor when you're "at rest", so it must be that one that's not OK with quick wrist motion.
I will add that I have really small wrists, so the watch fits snugly, but it's about as wide as my wrist, so maybe the fit isn't "typical". It's also very hard to get blood out of me, which someone else mentioned, so maybe my pulse isn't the easiest thing to read.
Anyway, if this is happening to you try twisting something around your finger. It seems like it might also happen if you were using a screwdriver, or any other light twisting motion.
