How do users know to hover over elements?

I realized something today. I was looking at a new icon at Airbnb's site, and wanted to hover over it to see if there was anything hidden that would expose on hover: there was.

My question: How do users know when to hover to expose elements? Do they? Is that only because I'm an experienced user (also a UX Designer by profession)? I'm interested to see if any study was done on this matter.