[Closed] Mysterious Pad Contamination

Three times in the last couple of months I have lost all function of the rear XT disk brake on my bike following a local, primarily street and towpath, ride (can barely lock up the rear wheel - braking simply has no effect). I replaced the pads and cleaned the disks the first two times and the brakes returned to normal function. I haven't fixed the problem yet for the third occurence of the issue but I am sure changing/cleaning the pads and disk will sort it out.

I am pretty confident the brakes aren't getting contaminated in my shed and can only think they are getting oil/diesel on them from the road during my ride. Is this likely/possible? Has it happened to anyone else? It is really getting on my nerves.