Someone entered our secured fencing system. Is this person dangerous? Help!?

I work on a farm, and it is surrounded by a 10 foot high fence. And at the driveway, there is this strong, iron gate, that only opens if a passcode is entered on a padlock. Well, only people that we trust, (3 family members of my boss) know the passcode. Well yesterday we were standing outside of the house when we saw a red pickup truck pull into our driveway. The gate was closed, but suddenly it swung open and the truck pulled in and started driving up and down the fields. So me and my boss hopped in our pickup truck and sped down the field after them. We followed their tracks around a corner when suddenly they disappeared and the truck was gone. We searched the entire farm but the truck vanished. We are planning on putting security cameras down at the fence, to see how they got in. We don't know anyone with that type of vehicle. What do you think they did to open it? Please help! We are trying to figure this out!! Thanks!