It's already 4 days since those 2 other answers were posted, and the fact that you commented 3 days ago so you must have seen them probably means you aren't satisfied with them, so I'll try to answer this myself.
As the other answer said, the gym was ignored until you saw it. The other one said that Ninatic added more gyms without announcement. I have another answer.
When I play Pokemon Go, I usually play it in my bus home from school. There was this one time, I passed a gym that was so easy to defeat, just a cp 47 (somewhere near 47) Butterfree. I defeated the gym (yay). However, when the battle was over my bus went too far and I thought it was rude to ask my driver back especially since there were other people in the bus, so I beated a gym without claiming it for Mystic :( but atleast I got experience. Perhaps the gym you saw was defeated but not claimed because the person who defeated it was in a hurry or went too far in a vehicle.
Hope I helped!