PetriMC

If it helps at all I had the exact same thing happen to me a couple of months ago, and like you it went away by itself. I speculated at the time (to myself!) that it might have had something to do with the particular lighting or reflections etc in the place I was shooting at that time, since it plagued me the whole time I was there regardless of switching on/off, pulling battery etc, to the point where the only solution was to turn the eye sensor off and switch manually - but as soon as I got out of there it was gone and everything was back to normal. Like you, I was putting nothing anywhere near the eye sensor, no straps cutting across it, anything like that, but just pointing the camera up and down was enough to trip it from LCD to EVF continually. Weird and annoying when it happens.

Suddenly the standard fitting screw-on diopter on the X-Pro1 doesn't seem like such a bad idea.