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Possibility 2, during the "free" bit at the start of a Shimano lever stroke, the valve between the master cylinder and reservoir is open. If your reservoir level was quite low and the seal at the reservoir port is better than the caliper seal, the pressure difference between ambient (inside the reservoir) and the lower pressure of the hold (higher than atmospheric at altitude, but lower than sea level) could force a little fluid past the caliper seal during a flight with up to 2 hrs cruising.
Don't know enough physics/engineering to calculate the likely seal leakage for a given pressure difference though.
Emyr
Howard
Was traveling with the bike last week, fine on the flight out and not so fine on the flight back. Had an improvised spacer (pound coin) between the pads to keep the spacing in the event that the lever is squeezed while the wheel is out.
Didnt notice that the inner piston leaked at the seal and only noticed it once I rode the bike - squealing under braking, brake loss etc... Touched inner side of rotor, confirmed that's there's a thin layer of oil, nothing on the outward facing side. Ditched pads, sprayed rotor with brake cleaner and wiped down caliper with rubbing alcohol put new pads in and all good now. Didnt even need a bleed.
What I think happened was that the lever must have been squeezed quite hard (probably when they chucked the bike when loading/unloading. This is with the lever removed from my bars and hidden (+ zip tied) in between the spokes of the wheel.
Anyone experienced something similar before?