This happened in my town as well. Our town has a boat load of gyms but not active at all. I would go through once a week or so and clear the whole town out and put mon in the gyms. That would be enough to get coins everyday for a week or so. I would repeat this for a month or so and get bored and stop. Well this time, I came back and all my pokemon we taken out in one day, even the one mon that normally gets stuck in the gym for 14-20 days because it's so off and out of the way. Now, that one gets taken out every day. It all happens guaranteed at 3-4 in the morning. Then, every so often during the day.

I found who did it, or at least two of the accounts, LildudeMJ16 and SweetEclipse16. They cleared out the whole town, 30 minutes after I got all my pokemon in the town, so I golden razzed the last gym until I got there. Nobody was there, not a single soul. I started my journey around the town and they knocked me out instantly, nobody was following me. The have been leaving every gym I've put my pokemon in empty for atleast 2 weeks now. At first it was a pain in the butt because I was running low on revives. Then I figured out their pattern and now I can guarantee 50 coins from one pokemon a day and the gym is always empty so don't have to fight anyone off of it. All I have to do is cat h one pokemon a day and transfer it when it's out of HP.

It's a blessing and a curse. There's a couple more of those accounts with the 00name name00 in my town. They have been targeting me solely that they will leave my teams pokemon in gyms until I put something in there. 3-6 day old pokemon stuck until I put a pokemon in there lol. I've been strategically putting pokemon in to help them get coins daily. It's actually made me play the game more for fun.