BMW Repair: coolant light/cold air question, stage unit, 328ci

Question
I have a 2000 328ci that has a problem with the coolant light going off/on and the heater blowing cold air when idling. I've been to the dealer twice for this: the 1st time they replaced the thermostat, the 2nd time they replaced the final stage unit and bled the cooling system. They said there were no leaks. The car runs fine for a week or two, then the same issue returns! Any advice?

Answer
Not sure what the "final stage unit" is.

Replacing thermostat makes sense, as does the subsequent bleeding.

When after a week or two the coolant light comes on and no heat, is the coolant low?  Likely there is a leak, either internal to the engine or external, perhaps at the heater.

If coolant is low after a couple of weeks, top it off and bleed it correctly.