Proximity sensor in the HRM and way to use it for isWorn?

Ahh, ok - I had assumed you just wanted to detect if it was worn or not - but yes, then an app that provides the activity detection might be best, and it can provide a library to get the current state. There is a method for apps to 'depend' on a module, so maybe in future someone else might want to contribute a different activity tracking app and people could switch between them.

Bangle.getHealthStatus provides a 'movement' field that might help too - if you can avoid leaving an accelerometer handler running all the time that would be preferable as it will hurt performance of other apps and kill the battery.