A number of years ago I had the task of culling the deer in a fenced woodland that was approx 100 acres.
I was putting the hours in and after several months it appeared that even when everything appeared to be going as planned, there were still deer appearing.
The noteworthy point was a lot of them had healed leg injuries.
What was happening, and this was a 2 metre high fence, was the roe deer were running around the outside, finding a high spot and using it as a point to jump from.
I saw roe jump the fence twice and was almost as surprised the second time as I was certain it would have been a one of the first time.
I suppose the point I'm making is we build fences, but nature always finds a way.