When moving foreign fleets leave sensor range, they remain on the galactic map at their last known location until game client is refreshed (not the on-watch update- they persist until the user leaves the page and returns or hits the browser refresh button).

If the intention is for these fleets' passage to be noticeable, a fleet that is no longer visible should have a different visualization (hollow icon, different-colored icon, or trail only). If the intention is for these fleets not to be noticeable, their icons should be cleared properly when they are no longer within sensor range.