Combine :after with :hover

Just append :after to your #alertlist li:hover selector the same way you do with your #alertlist li.selected selector:

#alertlist li.selected:after, #alertlist li:hover:after
{
    position:absolute;
    top: 0;
    right:-10px;
    bottom:0;

    border-top: 10px solid transparent;
    border-bottom: 10px solid transparent;
    border-left: 10px solid #303030;
    content: "";
}