I just saw a gym stay white for 3 whole minutes.

Other teams are not allowed to put something inside when someone has just defeated the gym. That could explain to some extent.

Okay.

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I'd assume that the player that took down the gym either:
- was waiting for someone else to take it because they did not want to immediately be identified.
- couldn't place a defender because they were a passenger in a car that is now out of range or already at max gyms.

Here there are some spoofers who take down gyms just to level up their badges. They don't put a defender since this would reveal them to any player who might actually be there.