I had the exact same problem with my 2012 Xt’s!
I got mine from Rose Bikes, Germany.
Sent it back under warranty and they sent it back with a brand new caliper – no explanation as to the fault?
I fitted the new caliper and headed off to Glentress for a run.
Took it into Alpine bikes to get it bled.
The Mechanic called me into the workshop and showed me that a tiny bit of fluid was seeping out where the two piece caliper joins together!
Only sent that one back to Rose last week, so no word yet.
I have also heard that the crimping machine that attaches the Banjo fitting to the hoses in the Shimano factory was faulty and they had lots of warranty issues with that –
Guess this could also cause fluid to get onto your pads, depending on the angle your hose meets your caliper?
Alan.