The EPA regulations require that the fuel tank is a sealed system so that no vapors escape. There is an entire system (Evaporative Purge) dedicated to that task. EPA regulations also require that the ECM (Engine Control Module) check they system for leak. When the right conditions are met IE fuel level between 1/3 and 1/2 tank, outside temp 50 - 90 etc. the ECM pulls a vacuum on the evap system and makes sure it holds a vacuum. This tells the ECM if there is a leak, allowing vapors to leak into the atmosphere. If it fails this check x number of times it turns the MIL (malfunction indicator lamp) on. This could be caused by a leaking seal, a torn hose, or a missing gas cap. If the problem is corrected the light goes out the next time the computer runs the test and it passes.
When this system first came out, salesmen and service advisers everywhere were telling anyone with a MIL light on "just tighten your gas cap the light will reset itself"
I suspect that the service adviser you were talking to (in your second question) did not know what he was talking about. If your car has a "check engine light" it has a computer and it stores a trouble code that it turned the light on for. It's crazy to say the computer is too old for us to know why the light is on.