Ok, this is strange.
So I turned off S Health (I wasn't able to find another place other than via the Application Manager) and the faint flash light stayed on. I restarted, it still stayed on. I even powered it off and it's staying on.
I don't have Google Fit installed, as someone else posted, I too get the 'this device isn't compatible with this version" message too.
If the unit wasn't sealed, I'd take out the battery just to see the light go off once .. :-/
Defective S6?
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also just found this similar report on XDA:
Flash always lit? | Samsung Galaxy S6 | XDA Forums
though again, google fit isn't (and can't be) installed on my S6 (fwiw, non-edge, 32 GB)
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finally, another observation, not sure if related, but I noticed that when I used the flashlight feature (which turns on this led permanently as I understand it) it isn't nearly as bright as the light on the S3.