With HPV+ it can happen that the primary may not be found. One thought is the body fought off the HPV disease at the primary site, but had a chance to metastesize, another is the primary dies off when the primary metastesises to the lymph nodes taking all the nutrients, blood with it. I guess similar to angiogenesis of something like that or the primary is too small to be seen on any scans, and only found when it is in the lymph nodes where it can grow much larger due to cystic fluid, rich in nutruents, blood. 80% of the time, an unknown primary is in the oropharynx, basucally the lingual and plentine tonsils, and reason they do a tonsillectomy, and radiate, usually both sides.

I have seen studies for one year overall survival with HNC HPV+ to be 100%, another at 3years overall survival was also 100%. I seen rates increased in one analysis after three years, but overall, is still low at 15% vs. non HPV.