When using the documents tab it is possible to drag and drop a document into the recent files list, but it doesn't do anything. I think that it would be cool if this closed the document (because the recent files list contains files that are closed). Of course it would ask to save first if the file was altered, and it is an easy operation to undo because now it is in the recent files list.
Right now, much of PyPE is a file drop target so that one can drop files basically anywhere to open them. Changing it so that that one place would close a document if it was dragged there would be a bit counter intuitive. Also, in the time it would take to drag a document to the close area, one could click on the document and use the 'close document' hotkey, which is ctrl+w by default.