White gyms

Because some1 took it down?
Sometimes raids pop up when its empty.

Sometimes when I have already collected the daily coins, I destroy enemy gyms just for fun and leave them empty. Keeps your opponents from getting coins and you don't waste revives.

One of the gyms I can see from my house used to battle and get left empty every night at the same time around midnight. This went on for a couple of months.

Eventually it stopped happening. All I can figure is it was some spoofbot that somebody left running.

Lots of people are already at the limit at 20 Pokemon in a gym at a time but still take down their rival gyms even though they can't drop one in themselves.

On a few occasions, I have taken over a gym but not placed a pokemon in it just for fun. Or because my phone ran out of juice.