My JQuery Plugin Appends "0" to the body element

I created a small JQuery Plugin and I'm using thickbox for popups.
I don't know where it came from but I have a "0" appended to the end of the body element.
I suspect that one of those outputs a success indicator maybe?

Is anyone familiar with this behavior?

Edit:

PHP

add_action('wp_ajax_product_picker', 'popup_content');
function popup_content() {
    iframe_header();
    echo 'aaa';
    iframe_footer();
}

JS

var currentPicker = {};

(function ($) {

    $.fn.pick_product = function (options) {
        var settings = $.extend({
            callback: function(){}
            }, options);
        this.on('click',function () {
            var url = product_picker.ajax_url + "?action=product_picker&TB_iframe=true&width=600&height=550";
            tb_show("My Caption", url);
        });
    return this;
    };
}(jQuery));

jQuery(function ($) {
    $('#test-product-picker').pick_product({
        callback: function (variation_id, size) {
            alert('aaa');
        }
    });
});

If you are using it on the front end of your site you have to use the

wp_ajax_nopriv_product_picker and also place die() after your function.

function popup_content() { iframe_header(); echo 'aaa'; iframe_footer(); die(); }