itch.io

Did you have the parking brake enabled while it was in/near the base?? The physics in the game are pretty weird when you go to sleep and fast forward time, it's possible that it just caught collision somewhere and gained speed and just flew away. Same thing happens with Kerfurs but usually they clip underground/underneath the base. 

Edit: You're also not even close to the day where an event occurs where that would be a possible thing that could happen (not intentionally, trust me it probably won't happen again) so I'll chalk it up to weird physics.

There are multiple spots where you can sneak your way past the fence, either through the huge opening near Yankee(?), or the main gate (jump on one of the waist high poles on the ground, then jump on the security post roof, and drop off the side) or certain corners and walls of the map that have a slight opening near the divider of the fenceposts, you can mantle over those.

Also make sure you bring the ATV keys bro I swear to god they will save you. Once you get the ATV make your way back to where you left the fencing.