It is stored inside the library itself, but it will be deleted when you quit FCP X.
You can retrieve using the Finder, but it is a little cumbersome.
Select the library, control-click, Show Package Contents; open the folder with the name of the event containing the project, then Shared Items inside it.
There will be one or more folders with names that look like gibberish. The most recent one is what you are looking for.
CAUTION: be careful not to mess with the structure of the library, as it can make it unusable. Only do this if you know what you are doing. It is after all not so much trouble to do a second export to file, or even better to create a "destination" that exports to master file AND youtube in one fell swoop.