Passive heart rate measurement in Apple Watch without green light?

I have not heard that the watch checks passively. It only check while the green lights are active.

 

Apple watch's pulse sensor has both green and IR LEDs according to information I've seen, so it's not really passive monitoring. IR LEDs are more power efficient than visible light LEDs, so therefore less of a drain on the battery.

 

I'm not sure if wOS supports any form of auto monitoring on OG watches. wOS 1 (the original release that came at launch and only lasted until autumn 2016) did brief 10 second or so periodic background monitoring while you were moving, and not running the exercise app IIRC, and which disappeared in wOS 2 as I also seem to recall. Not sure if it was ever re-implemented for OG watches in later wOS versions.

As you seem to be aware, S1 watches and onwards has some form of background monitoring, which I assume uses the less power hungry IR LEDs of the pulse sensor. Exactly how persistent it is I'm not sure; I haven't actually seen any sort of comprehensive breakdown of how that feature really works. I assume OG watches still has no form of background heart rate monitoring, to keep that feature unique to more recent watch models. Also, OG watch's battery really doesn't need the added strain!