I have had this problem for 2 years with my 2005 545i.

Coolant will empty every 2 months or so and I would have to fill it back up again. I took it to an indy shop in Dallas (Bimmer Autocare) and paid $1500 to replace all sorts of parts but didn't fix the issue. They wanted me to pay them AGAIN to look at it and replace more parts if necessary, so I said forget about it.
I moved to San Antonio and paid another indy shop (San Antonio Euro) $1100 to try to fix it - of course it didn't work. Took it back to the same shop, paid another $700 for them to look at it again and do a pressure test and they swear there should be no more problems. Well two months later, the coolant is gone. I now live back in Dallas and don't have time to drive back to San Antonio to have them check again - I'M SO FED UP!

My question is, would it be okay to leave the problem as is and just fill up every couple months? Will it cause further damage in the long run? I spent over $3000 already and do not feel like spending more.

Thanks.

 

I had the same issue too, and coolant would mysteriously go low without any signs of leaking or traces. I tracked it down to the center valley coolant pipe.

I read the thread below and he mention there's a weep hole that you can see traces of coolant leaking out. Which mine showed dried coolant trails, so I had a shop to fix this with the AGA pipe. It's been 4 months now, and coolant level has been steady as I've checked it every 2 weeks to make sure the problems has been addressed,so far so good. Thanks schpenxel for your thread, I know he post on here also but under another name.

Another one bites the dust... (coolant transfer pipe on N62) - Bimmerfest - BMW Forums

 

Yeah Phantazma I would have to agree. I had the same issue earlier this year. Every couple of weeks I was adding coolant but couldnt trace down where it was going. Then suddenly in June, I was on the freeway here in the atl area and I lost all of my coolant, had to pull off the freeway and sure enough it was the O-ring at the front of the coolant transfer pipe. The part it self was just a couple of bucks, but THE LABOR!!!! Fortunately I found an indy that did the whole job for me for $1800! So get it done before you get stranded like I was...