Here is where the "I think it was a fox" burrowed under the fence.
This one shows the light from outside the fence. Those fence boards were 4" below ground, so it suggests the amount of digging.
This one shows the scattered dirt inside the fence. That is a lot of scattering!
The spot is now filled with cat pee and poops and no sign of anything digging there since. Whatever it is, I just want it to leave my yard alone.