I have seen code like this in URLs where content from two different sources is used to populate a two window frameset. Seems that http%3a%2f%2f is just another way to write https:// that doesn’t mess up browsers.
Here’s an example: https://www.vrbo.com/global/siteFrame.asp?mainurl=http%3a%2f%2fwww.cascadecabin.net&returnurl=/50913
