What semi triangular component? Coolant leaks can be exasperating to track down. I'm chasing one in my X5 at this very moment.

When you took it to the dealer, did they specifically say the lower coolant hose was leaking? How many miles are on this car? If it's anywhere near 100K, it would be wise to replace ALL the cooling system parts (radiator, water pump, thermostat, hoses, pulleys, belts, expansion tank, and transmission thermostat (if it's an automatic)). As mentioned earlier, you DO NOT want to let the M54 engine overheat. I can speak from personal experience about the havoc that will cause.