There are 6 gyms in my area and every once in a while I will see that one of the gyms is completely undefended, which allows me to claim it. Why would this happen? In all the cases this has happened I looked around and no one was around, so I don’t think anyone was in the process of claiming it before I did. Are people defeating gyms and then just forgetting to claim them?
Some of my friends leave conquered gyms undefended because, at least where we live, gyms are un-defendable. Trying to defend just leads to a loss of potions. So, we go out on marauding missions and destroy gyms. Our calling card is that we leave it undefended (or have Pikachu be the defender). I just did this last night! 🙂 I’ve heard others do this to maybe send a message to Niantic.