How can I locate someone on yahoo w/o asking them? I want to sneak up and sup rise someone !?

If there was a way to track someone on Yahoo, then Yahoo would be liable. State and federal laws require all business and non-profit org to protect the confidentiality of their clients, customers, and employees. If you know their user name, type it on Yahoo or Google search. If you know a specific question they had asked, even if its already resolved, then type it on Y-A search, and see if he/she comes up.