I Know this thread is a bit old but this may help someone.
Motor 318i M43 although I would expect all 318's will be similar in this area.
I had a similar problem I could not find a very slow coolant leak. Ended up being the head gasket back of the motor below the black fitting for the 2 water hoses. Get a small extendable mirror and a torch / flash light and have a good look for dried coolant probably below the fitting.
I saw the coolant and guessed that the fitting had failed, $200 later new hoses and fitting I continued to loose coolant. Cleaned the area up with a cotton glove had another good look and could see a coolant bubble form slowly below the fitting where the head meets the block.
It's in the shop having the head and all seals done right now. Just a possabily if you can not see dried coolant anywhere else.