Originally posted by FissionChips:

Logging out doesn't clear the zone, and if you leave the zone the timer stops - so this could indeed happen every time they log on, if they haven't cleared it yet (by running out the clock in zone).

This is true. You can run out of the event circle in this case if you want since it can happen anywhere in the mountains or plains and not just at one of your bases. No risk losing buildings unless you are at a plains or mountain base.