How Did They Know?

Visited Morrison's Supermarket today.  Primarily because they had a '10p off' per litre, and my fuel light was glowing yellow :-)
Mrs MD suggested we had a quick walk around the aisles and get a few bits we needed.
The young lady on the checkout suggested we might like to visit the cafe and partake of a FREE hot drink and a mincepie - so we did.
As we entered the cafe I spotted this poster - Free........for all our Senior Citizens.
What intrigues me is, how did they know we qualified....... :-) :-)