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I'd take the demonic skin and eyes of the abyss feats (Fiendish Codex I). Combined, they give you darkvision out to 60 feet, a +2 bonus on search and spot, and +2 natural armor. (I'd forgotten about your darkvision requirement before, but the pixie idea can still be salvaged.)

The darkstalker feat (Lords of Madness) gives you a chance to hide from creatures that normally auto-detect you.

The quick fingers alternative class feature (Dungeonscape) for rogue reduces the amount of time it takes you to disable traps. Also, if you go rogue, you could take the demonic sneak attack feat (Fiendish Codex I) instead of the demonic skin feat. (No, demonic skin still seems better.)

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Pixies get those for ability modifiers and Greater Invisibility is always on unless they turn it off (a free action). That's remarkably tough to beat for any trapfinder and does make a fantastic rogue.

Artifcer's easily the best trap finder because of the infusions (and inherantly thanks to Spell Storing Item* you've got options... lots of them. It can really be TOO many if you don't know the magic system extremely well it can be quite overwhelming. I reccomend this class only if you're well versed in Spell Compendium and all sources of spells released after, as well of course as the spell section of the SRD/PHB. MAYBE if you've got someone at the table that will babysit you and suggest the perfect spell for the job you can do alright that way too. Where things get REALLY tricky is later on when you can use Concurrent Infusions (Magic of Ebberon), and create up to three such wands at once.
* (A 1st level infusion that lets you create a single charge wand, usable only by you that can duplicate the effects of ANY 4th level or lower spell)

Another trick of course with Artificer is their ability to enhance weapons and armor on the spot. Particularly the BANE enhancment on a box of (50) amunition can be devistating if everyone's got the same projectile weapon to use the ammo. At low levels this is basically the I win button.

As for build, you'll want to be Artificer 20 (eventually). IMO none of the PRC's you can do as Artificer are worth losing the artificer abilities. Feats are going to be your primary selection issues. Abilities that matter (in order) are Int (Skills, Infusions), Cha (UMD checks), Con (this always matters!), Dex (AC, Attack bonus with ray spells). Your Str and Wis are tirtiary to all that.

Finally... if you don't fit the bill of a spell book savy player, might I reccomend DO NOT PLAY AN ARTIFICER. I can't stress that enough. Just go with the Pixie Rogue. (Ask your GM if you can switch Dodge for Expiditious Dodge, Races of the Wild)

Use Tactile Trapsmith (Complete Adventurer), let your Int be OK but don't pump it (DEX will be your stat).

If you make your Int in the 18+ range: Dip 3 Swashbuckler , take Daring Outlaw (Complete Scoundrel)

Find your way into the Shadow Blade feat from Tome of Battle Bo9S (You'll need Martial Study for a Shadow Hand Maneuver with 0 Reqs. You'll need Martial Stance for either Island of Blades or Assassin Stance).

You'll be a badass trapsmith who can stab folk to death with a bobby pin. Max UMD and be sure a party crafter can make you scrolls of Grave/Golem/Vine Strike if you care at all about being able to help in a fight.

Rogue 3/Swash 3/Rogue+++
Feats look like
1 Tactile Trapsmith, Weapon Finesse, Expeditious Dodge (+2 more with Flaws, but I doubt you'll find good trades at 1 as a level 1 Fey, save Two Weapon Fighting)
3 Martial Study (Shadow Blade Technique)
6 Daring Outlaw +some to ref save and 1 or 2 d6 SA at any given level
9 Improved Two Weapon Fighting
12 Martial Stance (Assassin's Stance) +2d6 SA
15 Shadow Blade (Dex to damage, NOT precision)
18  ? Martial Study Shadow Stride( this is a fun pick just to give you a 50' move action teleport once/encounter)

Edits: Corrected for two errors