pb_close.png location

I have a number of sites where pb_close.png cannot be found when defined like this:
https://github.com/OneGov/plonetheme.onegov/blob/a586c33adedd502feb32d4e9316e1fd65f2339f9/plonetheme/onegov/resources/static/static.css#L1664

It seems like most of my sites actually have pb_close.png in the root folder (host/plone/pb_close.png).

Is there something amiss with my sites?
What's the reason behind the pb_close.png location as it stands now? Why is there a relative part to it?