Ai yooo, another water problem....
Always do the obvious first. Check all hoses and clips. Oh, not forgetting the ones going into the heater matrix. My friend (E36) had continuous water loss until he discovered one of these hoses was leaking. Like what BMW9700 said, check to make sure no water is leaking into the gearbox (usually applies to the E36.... affan66 experienced this, I think).
If still cannot find the source, do pressure test and hope it is not your cylinder head / block / gasket. Check if got water in your engine oil. Also check your water pump if in good condition.
In one of my water loss case, it was solved when I changed the radiator cap. Seems the cap was faulty allowing water to evaporate too fast.... now I hardly need to top up water.