I my sound like a jack-ass when i say this but from reading this thread i do wonder how your players manage to get any enjoyment from this. If you were to give them any check or advice of any sort all you have done is subtract from their gaming experiance. Will it give you a headache if they die? Yes. Will they have to roll new charecters slowing down the pase of the game that day? Yes. Yet in my honist opinion if players are not punished (fairly) for making their own mistakes it tottaly takes away any enjoyment of a role playing game and turns it into a social group that should be playing a ps3 rpg that has save points.
The crutch of "BUT BUT THE CHARECTER WOULD KNOW THIS EVEN IF STUPID HUMAN DOESNT" is not ok. To eliminate this excuse each charecter should be given a campagain relevant background that lists from them usefull information that can even strongly hint about safe places to rest in the area etc because then if they forget it was the players fault. For the game to be intresting and challenging it is the players you need to test.
On the topic of how monsters react to rest it depends on the intelgence of the monster but i personally am normally inclined to quite heavily punish pc's for resting in dungeons where i can unless they take logical percausions that make it plausable
I personally dont find wasting one of my too few gaming sessions making a new character because of a stupid or uninformed decision to be all that enjoyable.
I certainly dont find tossing out multiple adventures because my players made a stupid or uninformed decision to be all that enjoyable either.
Taking two seconds out to say, 'hey, that is a iffy spot to rest' is hardly going to break immersion.