It is supposed to reflect the number of, well, active HTTP requests, which for Shiny is usually one of: initial loading of the HTML page, downloading CSS/JS assets, downloading or uploading a file (via downloadButton or fileInput), downloading data from a DT data table or selectInput.
I don't know why this would be >0 while no users are connected. What happens if you terminate any active processes, hopefully it goes to 0 then?