Most post treatment scans are clear, in 90% percent range. I wonder why they did a RND, if there were no nodal involvement, metasteses, by the sound of your post, which may not be complete, but can answer that myself, and could depend on the primary tumor size, other negative factors, but these days a modified radical neck dissection, MRND, is done instead of a Radical to preserve one or more structures. Most recurrences occur the first two years, and recurrence or failure rate is high compared to non HPV, but that can also depends on the stage, TNM grading, and negative predictive factors, and can just see it in the five tear survival rates being less than 50%. Even if it does recurr, there still can be treatment options, and had 5 recurrences myself, multiple treatments, surgeries. I would take one day at a time, and so far it's clear, until proven otherwise.

I don't know what else to ask for if the PET is clear. Any further scan to prove otherwise, is not done, other than for a suspicion or scan schedule. Maybe your follow-up schedule.

Good luck.