Steve thanks for creating this query.  I was thinking about some single spots that might happen and be valid if the TxGrid matches the actual grid shown on QRZ and I found one such spot of of 10's that I checked and had no QRZ hits.  This was a 10 meter contact, and the band seemed pretty dead when I checked it later.  I searched back 5 weeks and K5UDO only showed up this one time.  I think that this situation is pretty rare though based on the number of calls that didn't have any QRZ or QRZCQ hits.