I have what appears to be the teensiest bit of oil ‘smear’ leaking from both of my two pairs of XTR trails – at the hose end. On dry dusty rides, it is visible as a thin film of black dust that collects around the hose end and runs over the calliper. I’ve also had similar honking problems that you describe. And with the honking comes poor power.
I wouldn’t have been able to spot this if it weren’t for the dust
I took to binding the hose end with a small bit of rag and insulating tape.. But that was probably a tad excessive!
I’ve since found that being really religious about really carefully cleaning the discs with methylated spirit immediately before each ride the problem is gone. The brakes sometimes honk a bit at the start of the ride – but any minor contamination seems to burn off by the first hill.
They are utterly brilliant brakes though. I can’t fault them for feel or power. Bags of the stuff.