Something you might want to look at:
I am not sure if this is the case with the 3 series, but if you have a slight coolant leak at your turbo lines, you won't even see it... you will just smell the vapourised coolant. It doesn't help that when this happens it also doesn't smell quite like hot coolant... It is also small enough that you won't get a low coolant warning for many many hundreds/thousands of kms.
In my case the smell was also difficult to pin down the first time since the fans would blow it all over the show when stopped... The first time this happened to me, I had the car on a lift to see if there was burning plastic on the exhaust from a bag or something I might have picked up on the highway. Even the tech initially thought it was coming from somewhere in the middle of the car LOL. You will have to wait for the fans to stop and listen around your turbo area for a faint 'sizzling' sound as it drips - if you can see the turbo and lines you might be able to see evidence of liquid falling onto it. Either way a pressure test of your cooling system will reveal it if that is indeed what it is.