This is my answer to your question, I am new here but I really enjoy this type of thing. In 2011, we had a very bad tornado come through our property. After that I have heard 4-5 new bird calls, which I have never before around here ( TennesseeTennessee), and 2 at night. If you think back to when you first heard this bird , had there been any bad storms where you live? I hope this helps, I've been an outdoorsman all my life and know how frustrating it can be to know how things are usually , and all of a sudden something new comes along and no-one else has noticed.
Thanks for the question,
GS