[pgsi]

Could be a hose, the metal hose manifold they have, the radiator, water pump , thermostat housing, coolant sensor housing or heater matrix.
You can purchase a uv dye kit which you put in the coolant, it comes with a uv light so you can search easier for leaks.
You will need to take the tray off to check properly.

 

[pgsi]

So basically you've just listed every part of the cooling system* - that's insightful & will be a great help in pinpointing the problem I:

OP:

Change the blue cap on the expansion bottle - the pressure release valve can open too early, common fault.

HH

* you forgot the aux pump.

The op asked about the various causes so i listed them for him to go through with a solution to help find the cause,

but as you have a crystal ball and know it all i assume your first guess will be correct.