I unplugged my eternal monitor while a Xojo project was displayed there (entirely).
Not even cost 2¢, nor a report on OS X, but the Xojo window was not going where one OS X user expected *.
The Window title and part of the window was located far above Xojos MenuBar instead of a standard placement.
I discarded the changes and re load that project to get the window at a normal location.
Did you ever saw a same behavior ?
On Windows XP, a window located on a unplugged monitor still appears there the last time it happens to me to make that “error”. That is why I wrote that sentence that way: on OS X, this is never a problem (OS X 'till 10.8.5; I do not know how Mavericks behave: I do not try that)