Most likely is a spoofer that doesn't want to get caught. Sometimes I'd take down a few gyms to burn through potions rather than discard them and leave nothing if I've collected my coins, but at the moment it's just too cold that I doubt that's the case. Many people are only playing for the daily bonus right now it seems.
Empty gyms?
I am experiencing something I have never seen before on my town and I have been playing since day 1. At this moment there is someone that has taken down all the gyms I can see from my house (4), they were 3 mystic and 1 instinct, all at decent level 10, 7, 6...But he/she/them are leaving them empty? They have been taken all down pretty quick and some of them are a bit far from each other, and now it has been more than 10 minutes and all are still empty. Has anyone experienced something like this? Is this a way of hacking/spoofing? Are they leaving them empty so I can't know who is the hacker? And if so what is the point of doing it?
Answers
I tear down gyms all the time and leave them empty. When you're already collecting coins from 10+ gyms daily, you can be selective about which gyms you even bother putting mons into.
Most of the time I just tear down opponent's local gyms without putting anything in afterward. I just enjoy the battle lol
Spoofers still put their pokemon in gyms and to even begin with, you can't really tell if their spoofing or not or even its even just 1 person maybe a group of friends but they leave them empty not cause their spoofers but to take down anyone that takes that gym. Think you get decent xp if its have the cp of the defending pokemon or just to send a message :).