Museum visitor numbers?

The news today reported that the national museums have hugely increased their visitor numbers since entrance fees were abolished. I am a regular visitor to the Science Museum, the V&A and the Natural History Museum and on not one occasion can I recall being 'counted'. No one clicked or clocked me in, insisted on giving me a free map as a way of keeping track, or electronically registered my footfall through the entrance. HOW DO THEY KNOW how many visitors they've had?

On the subject of museums, the Science Museum's Ingenious website has an interesting section on Health. It includes some wonderful pictures of artefacts from the Museum's collections, from a preserved genetically-engineered obese mouse, to a smallpox pustule gauge.