Pneumaticcraft has a helmet that does this but it is really hard to craft and since it's a piece of equipment, it has to replace your current, generally pricey, heavy armored helmet.
Plus, it has a google glass-like GUI that takes a while to load so it's even more impractical, since every time you put it on you have to wait for it to boot.

Here's a picture: http://i.imgur.com/20PiaXa.jpg

I was thinking of something a little more practical, like the magnets many mods already have, something that you keep in your inventory, that you could activate/deactivate by right-clicking it when it's in your hand.

Uhhhh, I'm not sure I like the idea of taking other mod functionality and just making it cheaper and easier. This behaviour leads to 5x macerators or other feature wars.

Also, I don't feel like messing with NEI API.

Hi ! I completly get your point but that's not really what I meant do ask for !

Just taking another mod's feature and making it cheaper would be terrible practice but the pneumatic helmet doesn't just do the item search thing, it does many, many more things, which is why it's expensive and bulky c:
It highlights you every single containers through walls hundreds of blocks around you, it can track entities, blocks, warn you of creepers behind you, it allows you to check the content of an inventory (even locked !) just by looking at it through a wall... Here it would just be a nice little convenience item for the un-organized folks. Something you can make rather early and all.

But if you don't feel like it that's alright thanks anyway