White box appears in center of page in Chrome

There is a small white box about 10 by 10 pixels that overlays all the page’s content when I use this plugin. I can’t click on this box to identify it by inspecting the elements and I’m not even sure it’s an actual html element. I’ve never seen anything quite like it. None of the elements in the code seem to correspond to it although the hidden map that opens when I click the image of the map is exactly below and to the right of this box (but invisible – I know from hovering over the html in Chrome’s Developer tools).

I’m not the only one who has had this problem, but there is no reply to the post here: http://wordpress.org/support/topic/little-white-box-in-center-of-page?replies=1

This box does not appear in IE, Firefox or Opera. Does anybody know what this could be?

https://wordpress.org/plugins/google-map-lightbox-popup/