The search results you see are there because they fall withing the set search radius in combination with the searched location
If I search for Pizza Rotterdam and zoom out, then Google Maps doesn’t suddenly shows pizza restaurants in nearby cities for me.
It’s on the todo list for the 2.3 update, so I will make this an optional feature.
What you need to do is use the zoom_changed event listener.
google.maps.event.addListener( map, "zoom_changed", function() {
// your custom code.
});
In the custom code section call makeAjaxRequest and pass a param when it’s done from withing in the event listener. If this is set, then use a make sure to modify the ajaxData values. So set the search radius to 10000 or so to make sure everything is included.
You may also want to disable the zoom button, when the new data is loaded to prevent people triggering several ajax requests at the same time when the for example zoom back from level 5 to 10 or so.