Where you park your car, if there were a slight drip of coolant from the front of the engine, would you have noticed it? That car is just about due for a new water pump if it has not already been replaced. My daughter parked her '97 on the street and never noticed the slow drip until she was driving home from college and the car overheated because it was low on coolant. It no doubt did most of its dripping when the car was moving.
The dealer certainly replaced the thermostat when he replace the housing, unless the mechanic accidentally picked up the old one and put it back in. They look rather shiny and new even when 14 years old. Although they are name brand thermostats, the thermostats in older 3 and 5 series BMWs seem to fail at an unusually high rate. The failure mode is “stuck closed”. I have never seen one fail after only two months though.
When I had one fail recently in a '97 328, the car would run unusually warm but not overheat when sitting still idling. If I pulled it out on the highway and put a load on it, the temperature gauge would go to the top within a mile.
Another problem you can have in a 14 year old BMW is crumbling plastic and internal leaks in the coolant external reservoir. When that happens, you can get air bubbles in the cooling system, and these cars do not like having air bubbles in their cooling system. I have replaced the external reservoirs in both my '97 328s.