I found this link helpful in explaining some things..don't know if it will answer anything for you, but worth a try.
http://www.paw-rescue.org/PAW/PETTIP...ip_Marking.php
I hear your frustration, but remember "spanking" even lightly won't really teach your dog anything other then to fear you. If you are unable to catch her in the act and correct it, I would try some of the other suggestions. Make sure you cleaned the spot with a good product to rid the smell. Good luck,
Kathy