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.
As you said, this is entirely possible. It could also be:
- Someone beats the gym with their last available pokemon, and has no way to revive their current pokemon.
- Their game may have crashed after taking a gym, and be unable to play
- May be due to bugs and issues
There is no tactical reason for this.
I don't see a reason to leave a gym undefended either.
There are a lot of factors that could stop you from claiming it though:
- Public transport: Maybe you can start a fight but move before you are able to claim the gym
- Running out of potions: The only requirement for a defending pokémon is 100% HP
- Server, GPS or battery issues...
As David Jacobsen pointed out in one of the comments, if you have defender bonus on cooldown then there isn't much to be gained for taking the gym.
Let's try listing some positives and negatives for taking a gym when you have your defender cooldown on.
- If gym is in remote area, hold on to gym to get bigger bonus next day
- Helping out other members of your team to hold the gym for a little bit longer to allow them to get bigger defender bonus (nice guy bonus :) )
- Creating name of yourself on the local area (having your pokemon on as many gyms as possible)
- Loss of health items after pokemon gets beaten (unless using fodder)
- Possibly have your best pokemon(s) sitting on the gym(s), unusable for your own attacks
- Leaving fodder pokemon awards almost free experience to opposing team while not contributing to defence if someone else wants to hold on to the gym
- Currently attacker has huge advantages over defender (dodging, choosing correct pokemon types, etc.)
In short: you are giving experience to opposing teams while netting only small benefits for yourself (or your team). Of course it's a whole different story if you are only high level person around...
Now that you need to use a Revive to heal your Pokémon who are kicked out of a gym, it's entirely possible that the battler is someone who simply enjoys battling, but doesn't want to waste their revives on healing Pokémon who were holding a gym if they've already gotten their gym-holding bonus for the day. Particularly as a role player (i.e., someone who plays Pokémon as an RPG, which the original game was) you could easily see someone defeating the gym to look out for their color but not want to waste resources on holding the gym themselves.
In addition to the above, there are those who don't care about holding a gym, but find it very fun to defeat a gym. It's not worth leaving a weak pokemon to defend a gym at this point, and they would want to keep their stronger pokemon to take over the next one. They wait for someone else to take the gym again so they can defeat it again. It's not always about being king of the hill, but it's fun to knock the current king down.
My first thought for why is experience. You get more for taking a gym than training it. I've done this before personally.
My friend also recently had an experience where a game shop's employees were continuously killing the gym (which could be reached from inside the shop). I think it was mostly to get people in the area to stay in the area (keep the players in the shop). I cannot guarantee that is the reason though.
People are missing the most obvious answer here: the person who defeated the gym was a passenger in a car/bus/train/etc. that goes past the gym. They clicked the gym right as they passed by, which allowed them to defeat the gym, but by the time the battle was over they were way too far away from it to actually put a pokemon in.