If the system has a small leak, you'd smell coolant, and a small leak always turns into a big leak in short order. If you are that concerned, then get a new cap and keep an eye on it. They probably didn't check the pressure valve in the cap. Hell, your lucky they were honest and didn't BS you into replacing something in the cooling system (like a cylinder head gasket or OFHG). There is no dealership in the world that would find a leaking cooling system and NOT recommend you fix it. If the dealer pressure tested the cooling system and it didn't leak, then it's not leaking. The reservoir cap can weaken over time and let coolant as vapor press escape. Cars use coolant over time, especially as they age the reservoir cap is a pressure valve, so when the coolant gets hot, it vaporizes and the vapor escapes. If it was just slightly less, then it's just a fluid temperature thing. How low was it? How warm was the coolant each time.

I think bmw dealer just did a pressure test Guess I should explain maybe take the bottom cover off take for drove then throw on lift to see if anything I'm not sure if I should bother taking back to dealer or use a Indy shop in my area that works on bmw Porsche etc that's pretty reputable. I give up fellas- checked after tipping off a few weeks ago, level is below the high level mark- I use light to check for wet areas on the bottom pan and dry as a bone.