Bike trail behind "no trespassing" fence; trail itself wasn't blocked.

I just got home from a short, experimental, nighttime bike ride. I was on an unpaved trail not marked on the map. At one point, without crossing any sort of barrier, I found myself behind a “no trespassing” fence. There was no sort of obstacle or “no trespassing” sign on the trail itself, so being behind the fence sort of shocked me. I carried on and got myself off the trail on the other side without running into anybody, though.

According to my dad, the trail was there long before the fence was built.

Questions: Should the trail and fence be marked on the map? If so, how? And did I actually break any laws?