Weird. I have never seen that before and I looked in my templates and it wasn't there at all for me. Then I created an ad just like yours, and it shows up. It's from here in the functions_ad.php file:
PHP Code:
// add a faux conditional before all the closing conditions to mark that we've shown certain ad already
$conditional_postfix = '<vb:if condition="$' . 'adsshown[] = ' . $adid . '"></vb:if>' . $conditional_postfix;
The number corresponds to the adid.