When mine leaked it was because the seal was worn
and when it heated up or twisted for an oil change it would drip. I also had stress cracked on the bottom of the expansion tank itself and it leaked their on my r56 when hot.

 

We just replaced my son's water pump last week. Yah COVID! We then discovered the "water pipe" behind water pump was leaking as well. The only way to replace this is to remove the thermostat housing and all the related bits you have been discussing. The job is tight but very doable with patience and a few common tools. The fact is there are 7-8 hoses and various connections that all come together in the same area. Trying to isolate which one is leaking ( behind that engine wiring harness ) i would think is early impossible. If my thermastat housing was not replaced in 2012 I would replace it with all the hoses.

In the future I would recommend that anyone who experiences a failure of any component of the coolant system just replace everything: Water pump, water pipe, thermostat housing and hoses (they are getting to the end of their servicable life now anyway). This was common on BMWs years ago and think it is applicable now.