This is just a "shot in the dark" because as previously stated, the cooling system is complicated. I had a similar problem awhile back and added some dye to the radiator that glows when subjected to UV light. No leaks could be seen. The dealer said I needed a new radiator and new water pump. I didn't jump at the chance to spend hundreds of bucks when I saw no leaks in either....
It turned out to be a 80 cent O-ring in what I believe is a heater hose. Pull the air cleaner and look on the left side of the engine very near the firewall. The hose comes thru the firewall and does a 90 degree turn into the engine block. There is a bracket held by two bolts and has an O-ring seal. It was leaking a very small amount but has not leaked in over a year and the radiator and water pump are still going strong.