My fiance still has this issue with his S60. He uses a Garmin HR monitor.
In past i was testing it. Without HRM, but using speed & cadence sensors. I guess the algorithm is depending on simple physics. Indoor running works cause the S60 can track the movement of your arm. So it will have a estimated distance (s). With HRM together it will produce more accurate values for calories. ( okay thats what it does not ...thx Daniel )
The Approach S60 does not use Firstbeat to calculate calories, and you will not receive calories for indoor activities because of this. It will require GPS data and will use distance and speed, alongside time and your user profile to calculate calories. Heart rate information will not be used to calculate calories.
Good to know. Firstbeat calculation is only in devices with optical HR sensors? If u read this: Calorie Terminology - Support Center its not mention that Firstbeat is the key.