It seem like she is sticking her nose into everyone's shoes when they come inside.
Is it everyone's shoes or just some people's? I had a cat who loved the shoes of one friend only. He did have cats.
My Tweety did that yesterday when a gal came round to see me , she spent a few mins walking around her shoes sniffing, the gal said she had a lot to do with horses ....
Perhaps she is curious about the odors on the shoes, something left there by another cat, a dog, or even from the person. One of my cats used to sleep on top of shoes, in positions that looked uncomfortable, but he clearly enjoyed.