N53 Coolant leak

My N53 330i is leaking coolant (surprise!). It leaks from full -> coolant warning in around 2 hours of motorway cruising, sitting at ~60-70mph, or in around 30 mins of driving moderately hard on backroads (revving to 4-5k & downshifting + rev-matching into corners). It doesn’t seem to be leaking when parked. When looking into the engine bay after driving there are no obvious wet patches.

There is no oil in coolant, and no residue on oil fill cap. I also used a head gasket sniffer when I bought the car and it was fine.

Any ideas? Only idea I have is that it’s leaking onto headers and instantly evaporating, but I can’t see how it would be doing that without any damp hoses.