That's a monster floor, on monster floors if you kill the last enemy and don't go down the ladder, or if you kill the last enemy on a spot that can't have a ladder, then a ladder will always spawn right at the entrance like that. It's supposed to make sure the player can always go down, although I guess it doesn't work that well if it's hard to see.The only reason I knew where it was going to be in your pic is because I spent 5 minutes looking for it in another similar post elsewhere a few months ago. IMO that should be a never-ladder spot just for those of us who totally pay attention and notice everything except for the intentionally cleverly hidden stuff which is like happening all the time nowadays.