Brother's friend checked me out in the grocery store?

So my brother used to be best friends with this guy, but they sort of split and went their own ways. They are both 26 years old. My brother's friend (we'll say his name is Alex) was standing by the little lotto machine in the entrance of the store. When I walked in, he kept staring at me so I smiled and kept walking. There was a mirror in front of me so I could see he was checking me out as I was walking away. When I had finished my shopping and was paying for my groceries, I looked up to see Alex watching me from two checkout lanes over. What the heck? He has NEVER looked at me like that before? Why was he checking me out?