I'm considering making traps in areas I won't be going to very often with water channels washing items to a central collection area, but I'm unsure whether those traps will catch anything. I've noticed that mobs spawn alot in areas where I currently am, and don't appear as often when I go exploring
EDIT: let me reword that.... do they spawn in areas that you is not in your fog field of vision?
Mobs spawn anywhere within an area greater than 10 blocks and less than 40 blocks from your current position. This also applies to hollow areas that might be below you. Anything outside that usually won't have any spawns, and will usually despawn anything that was previously there. Things which get close enough to you to begin following however will usually persist though, so if you hear mob sounds from below while wandering around, you should try and deal with them asap, or staying away so that you don't have a large cluster of stuff in that area.
The main reason why traps fail is because its hard to anticipate where things will spawn unless you specifically look and pay attention, and because you have to be a certain distance away from that spawn point for anything to appear. This is why I've started just placing burning logs around everywhere, and have pretty much given up elaborate trap pits. There's usually enough drops lying around from burned enemies come morning to keep me happy, and I no longer have to waste weapons killing pigs and cows.