What does “suck a salt grain off a beach” mean?

In association with my question of the usage of “blood-dimmed (flood /tragedy) in Maureen Dowd’s article in New York Times- http://www.nytimes.com/2013/06/09/opinion/sunday/dowd-peeping-president-obama.html?hp - there was the following statement:

“It was quaint to think we had any privacy left, once Google, Facebook, Twitter and Instagram braided themselves into our days and nights.
As Gene Hackman, playing a disillusioned N.S.A. analyst in the 1998 movie “Enemy of the State” put it, the agency has been in bed with the telecommunications industry for decades, and “they can suck a salt grain off a beach.”

Though I surmise “they can suck a salt grain off a beach” figuratively means to obtain private information of people indirectly, not through a direct contact (to the sea - people) I’m not sure of.”

What does “sack a salt grain off a beach” mean? Is it an idiomatic expression, or twist of a saying? If it is the twist of, or borrowing from something, what is the original source? Why did Dowd put the phrase in parenthesis (correction: quote)?

A little more context:

BRILL: They can tap anything as along as it’s an airwave intercept. Cellulars and pagers your kid can do. Hard-line calls we'd pick off the relays as they were being fed into ground cables or fired up to the SATs. We’d suck in everything. All foreign, most domestic. Domestic was my group. Druggies, radicals, loud-mouths. Anyone we wanted.
DEAN: How’d you have the manpower to--
BRILL: Meade has 18 underground acres of computers. They scan every phonecall for target words like "bomb" or "President". We red-flag phone numbers or voice prints...whatever we wanted. When the computers found something, it was bounced to comparative analysis.
DEAN: Jesus.
BRILL: That was twenty years ago. With digital? They can suck a salt grain off a beach.

It’s a metaphor: the NSA’s capability is analogous to being able to survey an entire beach and identify a specific grain of salt as being of interest. ‘With digital’, they have no need to ‘suck in everything’ and analyze it manually, as in the 70s; they suck in only the specific information they need.

It may be hyperbole; or it may not. In the 80s I had a friend ‘in the intelligence community’ who dealt with satellite imagery. He was fond of saying “I am not telling you we can read a license plate on a Mercedes in Red Square. I’m not telling you that.” That was a quarter-century ago . . .

The phrase "they can suck a salt grain off a beach" is, as far as I can tell, neither idiomatic nor a reference to something - it is rather a vivid, if subtle, description of the precision with which the NSA can (allegedly) work - the beach is made of grains of sand, making a single grain of salt very hard to find, but they have the technology and personnel to do it.