In assessing negligence, what duty of care is owed by landowners to undiscovered trespassers?

Hypo 1: Sam buys a parcel of land in the middle of the Alaskan wilderness, so far remote that it doesn't show up on a map and requires a 2-hour helicopter ride to reach. While there, Sam decides to create an animal trap consisting of a 30-foot deep hole hidden under branches and leaves. One day, Bob gets lost in Alaska and somehow finds his way onto Sam's land, falling into the animal trap pit and breaking his legs. Result: Sam had no reason to expect anyone to be on his land, and he owed Bob no duty of care. It would be ridiculous if the law required Sam to post a warning sign, or build a fence, around a pit located on his land in the middle of nowhere. Bob should be more careful.

Hypo 2: Amy lives in the suburbs of a large city. She is attempting to install a pool in her backyard. Little did she know, Bob likes to sneak into her backyard to steal her avocados at night. When he jumps over her fence that evening, he doesn't see the large hole dug in her backyard and falls in, breaking his leg. Result: Amy had no idea Bob was trespassing in her backyard, so she had no reason (or obligation) to consider whether the hole in her backyard was a danger to random trespassers.

Hypo 3: Bob hates how local teenagers use his backyard as a shortcut while going to school. One day, after Bob finishes cleaning his bear traps in his backyard, he decides to leave them out in the sun to dry. Soon after, a teenager cutting through Bob's backyard steps on one of the traps. Result: Landowners owners do not owe a duty of care to undiscovered trespassers. However, here, Bob knew that teenagers often trespass on his land, and so he cannot standby and do nothing while knowing that people regularly trespass through the route where he has left his bear traps.