I was pedaling along and counting down the minutes when the seat vanished from beneath me and I followed with a resounding thump. I shut things down and took a look. The seat is held in position by a large screw attached to a giant knob that runs through the matching hole in the seat shaft. That screw was on the floor.
daisy_knotwise was in here sewing while I was at the baseball game Wednesday. I know that little girls were messing with the exercycle, because some of the buttons on the trip computer had been pressed, putting it in a different mode. I simply put it back in the right mode and continued.
I strongly suspect that one or more little girls were busy turning the large friendly knob on the end of the screw that holds the seat up. And two days later...
Whee! At least there appears to be no damage to either me or the bike.