On the roof of the concession stand.
We had a softball game at said field on Tuesday night, and the first thing RNR, Bonus, and I did was to go behind the concession stand and gaze up at the roof. Next time we go, I will take a camera. I know. I should have had a camera with me to record Miss Proper's softball heroics, but I am not a camera mom, just like I was never a bib mom, a wipey mom, an extra-pair-of-clothes mom. It just doesn't occur to me before I leave the house.
I'm pretty sure it has melted onto the green tin roof, which means it will most likely be there until they tear that building down. All of the printed-on color has faded and the yellow rubber has a slighty grayish tint now, but it is definitely RNR's flipflop. It is still there. Go look for yourself.