I don't think anything had gotten under the brake pad. If it were something under the pad, there would have been a grinding/grating noise. If it was hilly terrain, the rotors could have overheated and caused the vibration, but if they are warped, they do not straighten out theirselves. It is possible something got between the caliper and the rotor for a while and caused this. It would have had to been something that stuck to the rotor for a while and vibrated the caliper piston back into caliper bore. Maybe ran over a rag, plastic or similar product. Possibly small piece of wood. You got me thinking on this one. Since it corrected itself, something had to wedge in there to cause the problem. Is this your normal everyday driver? If it is, pull wheel and look for any indication of something being there. Such as scratches, discoloration, clean spot where everything else has rust or dirt on it. If it were anything mechanical, it would have to be in the hub assembly. But then again, it will not fix itself. Got gray hair now, and you trying to make me lose it.
You are correct in assuming something had caused the brake piston to be forced into the caliper bore. This would count for the extra fluid in master cylinder. But, I can't guess as to what is would be. All I can think of is road debris that fell out on the second stop. or when you pumped the pedal and forced piston back out. If you find the explination, please let me know.