FACT 500K – but is it?

Apparently FACT had its 500,000th visitor recently which is great, I’m a member and love the place. I reckon I’ve probably called in about 1,000 times since it opened in Feb 2003 so I’m wondering, does that count as 1,000 visitors or just 1?
It can’t be the latter surely. Unless they have some super surveillance equipment that identifies everyone that goes through the door and knows whether they’ve been there before. Its not a website, I don’t have my IP address stuck on my forehead. And is it every visitor to the building? The staff, expecially the security guard go in and out of the doors several times a day. Or is it visits to the exhibitions, cinemas, bar and cafe? Whatever, I don’t see how it can be any more than an ‘educated’ guesstimate. Why don’t I just ask them?