One major player is often overlooked as something of a dark horse in the industry.


A dark horse

Meaning

Someone who emerges to prominence; being previously little known.

Origin

This was originally horse racing parlance. A dark horse was one that wasn't known to the punters and was difficult to place odds on. The figurative use later spread to other fields and has come to apply to anyone who comes under scrutiny but is previously little known.
http://www.phrases.org.uk/meanings/108200.html

"The Google Sneak Attack: building a software giant under the radar."

Or perhaps, less likely, "a sleeping giant"

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Note added at 25 Min. (2012-12-11 15:59:37 GMT)
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Possible translation:

"One giant has flown under the radar."

Time magazine as well as some open software people seem to like the term "unknown giant". However, some of the other answers are quite good, too.

As I'm trying to sell "unknown giant", though, there is this article from the New York Times about Lenovo, a computer manufacturer and they head their article with "An Unknown Giant Flexes Its Muscles".

should be possible too...

You already have a couple of good options on the table (I like the hidden and unknown giants the best), but just for kicks I'll throw in a couple more.

"Secret giant" sounds more natural to me, so I was surprised at how few Google hits it got -- and the top one comes from a Brazilian source. I still like it though.

"Harald has developed into a 'secret' giant of the South American chocolate industry over the past years."
http://www.buhlergroup.com/global/en/about-buehler/news/arch...

"J.S. Cardone, the screenwriter, has one of the most thrilling IMDB pages ever recorded. A secret giant of Hollywood."
http://gawker.com/5382765/

"Stealth giant" I find a bit more awkward, but the NY Times liked it enough to use it in a headline:

"A Stealth Giant in the Gun Industry"
https://www.nytimes.com/slideshow/2011/11/27/business/27gun-...

"VF Corp. is something of a stealth giant, a company that touches your life in dozens of different ways without your ever being aware of it."
http://books.google.com/books?id=DidZAAAAYAAJ&q="a stealth g...

And it's also similar to "stealth startup," which is the right context (http://en.wikipedia.org/wiki/Stealth_startup). On the other hand, "stealth" as an adjective often has negative connotations (stealth candidate, stealth tax, stealth juror).

Another option

unnoticed industry leader
inconspicuous powerhouse

Seit langem wird in DE beklagt, dass das Land nur ganz wenig große IT-Konzerne hervorgebracht hat. Ein heimlicher Riese wird dabei gern übersehen.

a big company in this market but not seen/noticed/suspected as such in this market.
They're not known as IT powerhouse but they are.


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Note added at 1 day8 hrs (2012-12-13 00:09:16 GMT)
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unsusoected: meaning 2:
meaning 2: http://thefreedictionary.com (= not known to exist); alternative: inconspicuous ..., little-known ..., (yet) unheard-of, an ever-so quiet leader/powerhouese/giant
a giant unbeknownst to anyone