I bet this is what happened... some other applicant called in to check the status of his application. She put him on hold to go check. She found it, came back and picked up the phone. Then you asked about the status of your application and she thought "dummy, you just asked me that" and then told you that it was complete. I have no idea how they would have actually known it was you, and I would assume they thought they were talking to someone else.