Those "illegal guns" were most likely perfectly legal firearms, but in the hands (or more likely, pockets) of people who possessed them, or were carrying them in an illegal manner...
BTW, Tysons IS posted, but not clearly. They do not have the highly visible "circle slash gunbusters" type signage. They have "rules of conduct" posted, and way down at the bottom, in tiny 12 pt type there is a line prohibiting guns, knives, or any sort of weapons.
Tysons is posted, but not in an obvious or clearly visible manner--which is becoming a disturbing trend with malls all over the country. You have to first realize that those "Code of Conduct" signs are about prohibiting something we all do, and then take the time to read it...
It's like these signs are designed to be ignored (they are not prominently placed, they are not eye-catching, and they use tiny type on low-contrast backgrounds, so they are easy to miss and difficult to read.). They are designed in a manner that does not make you want to pay attention to them, almost like they are hoping you'll ignore them, and then they can "catch" you carrying against their policies...
This isn't just "bad graphic design", folks. These signs are symptomatic of the insideous, duplicitous, and deceitful tactics that the anti-gun British and French multinationals who own most of the malls in the US are willing to go to to mess with us...
Boycott malls that are "posted"...