What is this time/weather penalty drain that drains your sneak? I noticed a drain spell on me that drained my sneak, but I just shrugged it off, because I figured it was from a fight, and I waited out the drain. Then it comes again, and I know it didn't come from any NPCs this time since I wasn't involved in any combat. I look at it, and it says time/weather penalty, and it causes sneak to be drained.

Has it always been there?

Probably one of the Lighting Mod modules.

Ah, I installed that a couple of days ago... Oh I see, it's part of the whole being able to be more stealthy in the dark thing. Thanks. Not sure if I want this... I do like the lighting effects though.

There is a http://planetelderscrolls.gamespy.com/View.php?view=mods.detail&id=609 if you only want some of the features. The readme for The Lighting Mod even mentions it, as well as the sneak drain. Be sure that you know what a mod does before you add it to your game. Be sure to read the readmes. You can get serious problems if you don't.