many thanks!

ModelAdmin automatically creates the Contact GridField for the LeadsAdmin. Your codes overwrite that gridfield.

I think it is a 'bug' that GridFieldAutocompleter doesn't look into the $searchable_fields of the Contact object to find out what searchable_fields are. It only checks if the Contact object has a 'Name' field or not to decide whether the Contact object is searchable or not.

Although your solution works but i think the perfect solution is to update The GridFieldAutoCompleter. Hope the SS team can look into this?

regards,
Ben

Yep at the moment it really isn't obvious what went wrong when the "No searchable fields could be found for class ##" error occurs.

I stumbled over the same bug but after some trial and error I decided to modify the GridFieldAutocompleter in the core. For anyone interested:
framework/forms/gridfield/GridFieldAddExistingAutocompleter.php line 260

Old code:

	protected function scaffoldSearchFields($dataClass) {
		$obj = singleton($dataClass);
		if($obj->hasDatabaseField('Title')) {
			return array('Title');
		} else if($obj->hasDatabaseField('Name')) {
			return array('Name');
		} else {
			return null;
		}
	}

New code:

	protected function scaffoldSearchFields($dataClass) {
		$obj = singleton($dataClass);
		foreach($obj->searchableFields() as $k => $v){
			$searchable[] = $k; 
		}
		return $searchable;
	}

Thanks for the fix PmKoetsier,
would be great to submit this change to the core files...

I just created an updated function that fixed the search_feilds bug, and also sets the results format too - or else all the search results end just just as an ID

protected function scaffoldSearchFields($dataClass) {
		$obj = singleton($dataClass);
		if($obj->hasDatabaseField('Title')) {
			return array('Title');
		} else if($obj->hasDatabaseField('Name')) {
			return array('Name');
		} else if($obj::$searchable_fields){
           $this->resultsFormat = "$".implode(", $", $obj::$searchable_fields);
           return $obj::$searchable_fields;
        } else {
            return null;
        }
}