To be honest I'm not sure. Looking at the documentation the search event should work.
Edit It really helps to read documentation. :D
Search method.
Triggers a search event, which, when
data is available, then will display
the suggestions; can be used by a
selectbox-like button to open the
suggestions when clicked. If no value
argument is specified, the current
input's value is used. Can be called
with an empty string and minLength: 0
to display all items.
$( ".selector" ).autocomplete({
search: function(event, ui) {
$.ajax{
//Your ajax parameters...
success: function(data) { //No idea what format your data is in...
if(data['status'] == false) { //there is no result
//return your data.
//Trigger the events you want if the item does no exist.
}
else if(data['status'] == true){
//return data normally.
}
}
}
}
});