90% of unknown primary or occult primary, is in the oropharynx in the tonsil, mainly lingual or palantine, and base of tongue, as Bart mentioned. For OP usually a tonsillectomy is performed, sometimes bilaterally, including a neck dissection on the positive node finding side. Also as Bart mentioned a PET/CT scan is done, and a panendoscopy, under anesthesia, with biopsies. which was probably done during yout surgery.

The Tonsil, and BOT are difficult areas to detect with all the crevices, and the BOT more so due to its deep musculature, and both likely to metastasize due to the lymphatic tissue, and the tonsils are part of the lymphatic system.

There are several thoughts of unknown primaries, one if it was HPV related, it is usually small, and the nodal metastesee grows larger faster due to the blood, nutrients in surrounding tissue. The primary is small, undectsble by any scans or was missed in the biopsy. Another is when the primary metastasises, it takes all the nutrients, and blood with it, and primary dies off, and if HPV related, the body could have fought of the disease.

Most likely they will radiate the oropharynx as if the primary was there, in the waldeyer ring.

Good luck.