I have a pretty barebones site, with around 5 different levels and I am using paypal express as my payment gateway. No addons or custom code. As for when it is occurring it seems to be completly random. I made sure nobody was on the site and nothing was happening but still my debug log was getting larger. I also have this error if it helps:
[22-Apr-2014 12:15:35 UTC] PHP Notice: add_option was called with an argument that is deprecated since version 2.3 with no alternative available. in /var/www/wp-includes/functions.php on line 3008
