Honestly, not much you can do.
I could be wrong, but from what I read from one source some time back, unlike Japanese cars where complete sealing takes place, European cars are different. Even if you set it to recirc, the flaps will open every now and then to allow fresh air to come in. Something to do with their regulations.
My E90 was like that from day 1 until the day i sold. Nth to do with the sealing. What i did was to tape some duct tape across the inlet. You can find the inlet after removing the cabin filter in the engine bay. A very ghetto thing to do on a car costing >200K at that time, but it solved the problem.
