Massive coolant loss?

[silver141]

I put on a post a couple of days ago regarding a water pump issue for a 1999 318(e46).However things have now got worse and I am only managing to travel about 5 miles until my coolant warning light comes on.Ive had a good look this morning whilst on my drive with the engine running at normal temperature but cant see anything aspecially with loosing this much coolant I thought any leaks would be very easy to spot.Is there anything that could be leaking whilst the car is being driven and would not leak whilst the car is stationary?The car seems to running ok and the temperature is bang on halfway like it should be?Any ideas?

[hoppo4.2]

Look under the mats in the front. If the carpet is wet your heater matrix is gone.

[TROOPER88]

The head gasket has most definately gone.

If the blowers are working as they should I would say HG for sure.

The water is being lost through evaporation through the exhaust.