It was wadded up and stuck in the back of his nostril. Apparently he had eaten some grass then sneezed and it shot up into his sinus cavity from behind his soft palate. The Vet said the team was amazed when he snagged the offending greenery and it just kept coming and coming.
Obi is still pretty wiped out today. I was told they had to give him a shot of Acepromazine after the procedure because he became very anxious as the anesthesia was wearing off, and they didn't want him to irritate the tissue more by barking and sneezing.
I am SO relieved that there was no need for a biopsy. Thank you all for your good thoughts!