// include auction ID in bid note email subject (or any other product info available)

add_filter('woocommerce_email_subject_bid_note', 'tgg_woocommerce_email_subject_auction_win', 1, 2);

function tgg_woocommerce_email_subject_auction_win( $subject, $product ) {

	$subject = "Congratulations!! You have won auction ". $product->get_id();
	return $subject;
}