If you put your watch on a table and don't move it, then the leds should be off.
But if you move it, even when not on a wrist, then the leds will turn on to check, if nothing to read, then leds will quickly switch off. If you keep moving it, then the leds will turn on again.
In such case, you can only manually turn off Heart Rate setting when you're not doing an activity. And turn it on again when you need it. Activity -> Menu ˅ (down) -> Settings.
