Hi, just a suggestion, use my free plugin "Automatic modal advertising" on market. Open subscribe.php file, the initial code have a conditional to appears modal window only in one page results and avoid all other pages on search. Don't know if exists any other way to do it, but for me results well on this plugin.
Change the $iPage variable to value 1 :
$iPage = getPreference('ama-ipage-alert-form','automatic_modal_advertising');
If you feel some difficult to do it on your theme, I can try help you with that. Try a way to contact me, or from market, the plugin is free, so I think you can open a support ticket after your free download.
Regards