Maxperson
Morkus from Orkus
Why didn't you initially describe that trap when you described the room? If I auto-succeed, why wasn't I told as soon as I could see it? And, note, an auto-success means that no matter what I rolled, I'd succeed anyway, so, again, it shouldn't make any difference whether I rolled or not.
Because it wasn't in plain sight, but as soon as you look behind the couch it's right there, plain as day. No roll required and not a part of the room description.
But, the player cannot ever determine that. The player never knows if an action is in doubt or not. If it wasn't in doubt, no matter what the roll was, it succeeded. If the climb DC is 5 and I have a +4 climb skill, it doesn't matter if I roll or not, I move half my speed up whatever it is I'm climbing.
Players also cannot initiate rolls in 5e.