Water pump?
One thing to check is your water pump.
I had a recurring leak that became progfessively worse. When I had the roadworthy certificate check for registration I asked the mechanic to check out my cooling sytem for leaks. He said that my water pump had a leak and quoted $700 to fix (Aussie $$$). I bought a new watre pump from BMW along with 2 bottles of BMW cooant (don't use anything else) and replaced myself in about 2 hours. Toatl cost came in under $200 (Aussie $$$). Water leak gone.
That is one possible cause.
You'll need to check around your water pump with the engine warm. Use a torch and a mirror to help.
Good luck.
Cheers,
Trevor (Sydney)
E46
00' 323iA (70,000 kms)