Stealthy Colossus

Unknown to them all, The Minotaur is hiding in the shadows behind her. This is not an easy thing to do when you're 10- feet-tall and your head is a cow.

—Cracked.com

A Stealthy Colossus is a character or machine that is ginormously huge (relative to their size category, anyway) yet somehow manages to stay undetected whenever they want. While a Barbarian Hero might still conceivably sneak up on someone, these guys go even further. If the setting is comical, they will tiptoe behind their prey. Otherwise, they will enter the scene in a fittingly dramatic fashion, possibly with a Surprisingly-Sudden Death.

Related to Impossibly Graceful Giant, Acrofatic, With Catlike Tread, and possibly Offscreen Teleportation and Suspiciously Stealthy Predator. Compare/contrast Highly-Visible Ninja.

Examples of Stealthy Colossus include:

Anime and Manga

Gundam Wing has Death Scythe, which has the ability to cloak. When used in deep space, it fades Predator style and blends seemlessly with the surrounding field of stars; effectively becoming invisible... and makes no sound.
There is a One Piece character named by Sanjuan Wolf, although his nickname is Colossal Battleship. He is at least 180m tall and hid near a big battle and only a handful of people, all of who where his allies, knew he was there. What makes this even crazier is that this setting has Observation Haki which is an additional sense people can gain get, and there would have been a bunch of people at the battle with it.

Comic Books

In The Middleman (comic and series), El Comelon ("the Glutton") is a Mexican Wrestler-Assassin who weighs 400 pounds, but has been known to "squeeze through spaces the diameter of a tennis racket."

Fan Works

A Keychain of Creation fan comic had the Kukla sneaking up on someone. The Kukla is a dragon the size of a mountain range.
MegaZone of Undocumented Features, despite being a hulking bald guy over six feet in height, usually dressed in a black Badass Longcoat and combat boots, has an unnerving tendency to accidentally sneak up on people.

Film

Zilla from the 1998 remake of Godzilla is somehow able to sneak around and hide in the middle of New York City without anyone noticing him, for the most part.
Transformers: The film (almost) makes a joke of how easily massive robots can hide.
Megamind has the villain in a multi-storey mech at one point, sneakily hiding around corners while the Face Heel Turn hero he created flies by at top speed.
In Cloverfield, the eponymous multi-ton, many-legged, several stories-high monster sneaks up on the protagonists and bites the cameraman in half.

Literature

In The Wheel of Time, one of the characters that travels with Mat for a while is Chel Vanin, an exceptional former horse thief and poacher who also happens to also be particularly fat. Mat notes multiple times just how remarkable he is, given his fatness. Indeed, Mat judges him to be so good at what he does, he makes him the head scout in the Band of the Red Hand - that's right, a balding, fat head scout.
In Myth Adventures, Skeeve has Chumley sneak up on and kidnap his apprentices, to teach them about judging people prematurely.

Tabletop Games

The Munchkin Fu set for the Munchkin card game has a mecha ninja named "Fifty Ton Master Of Stealth". Also, the Ninja Giant from the "Need for Steed" expansion.
Dungeons & Dragons third edition bestiary depicts a gold dragon sneaking behind some orcs - possibly justified by magic.
In Warhammer 40,000, General Ursaker E. Creed is capable of applying the "Scout Rule," to any unit. This means that he can take any unit on his side, and move it into flanking position of the enemy army. Instantly. This would be an inversion if all he could move are scouts, but one can perform this rule with Space Marine squads (gigantic, ten-foot-tall, multi-ton killing machines), tanks, and two-hundred feet tall mecha, appearing out of freaking nowhere. It has become a Memetic Mutation that he is capable of infiltrating any unit to any point anywhere, at any time, ever. This is known as "Tactical Genius".

Video Games

In the ork ending cutscene of Dawn of War 2: Retribution Captain Blutflagg manages to sneak up on an inquisitor, despite being about a size of a dreadnought. Spookoms might also qualify, being an ork stealth expert (he is an ork "kommando" nob and so about eight feet tall and over four feet wide).
The guy from Mark Of Kri (video game, sequel was Rise of the Kasai) is a huge quasi-Polynesian guy that's easily double the size of his enemies. Sneaking around and stealth-killing is a major part of his gameplay. However, his huge size does come in handy: he's able to stealth-kill two guys at once.
The Devilsaurs from World of Warcraft were notorious for being able to sneak up on players despite being T. Rex sized dinosaur-like creatures.
- The Fel Reaver was similarly notorious, despite the ground quaking from its steps. Someone who wasn't paying attention would notice the shaking, dash wildly in a direction they can't see the reaver, and discover too late that it was approaching them from an angle and they've run right into its aggro radius.
A mechanized example: aerial fortress Gleipnir from Ace Combat X: Skies of Deception is enormously huge yet its cloaking system allows it to make stealth incursions into enemy territory with impunity.
The roadies in Brutal Legend haul enormous piles of amps on their backs to battle... but are completely invisible unless they attack. Why? Because they wear black and in the Heavy Metal-inspired setting of the game, that works as a perfect Perception Filter. Eddie, being a roadie himself, can also become invisible by jumping on top of said amps.

Web Comics

From The Adventures of Dr. McNinja, there's Mongo the Uberninja. He's a 7-feet-tall, colossally-muscled giant who habitually dual-wields chainsaws (that is, chainsaw-chucks)... but he's still able to sneak up on anyone, even the main character, because... well, he's a ninja!

Western Animation

Gossamer, the Mad Scientist's monster in Bugs Bunny cartoons.
Parodied in an episode of Invader Zim with the Megadoomer, an impractically large robot with a "stealth" mode. It turns invisible, but it trails its visible cord behind it, leaves enormous footprints, and doesn't cloak the rider.
The Megas XLR episode "Breakout" has a large group of mecha-sized alien criminals released at one point. One of them appears to be a martial artist with a cloaking device, which he uses to move around silently and use hit and run tactics on Megas. This being Megas XLR, he is defeated when Coop fires missiles in all directions at once.

Real Life

Despite their massive size, elephants are surprisingly stealthy. They actually make very little noise when walking and have been known to sneak onto farms and eat the crops without being noticed.
Any large submarine. That's kind of the point of being a submarine.