Gaseous clues uncover the cause of recurrent urinary symptoms

Bob is a 74-year-old retired veteran who presents with a three day history of dysuria and urinary frequency.
He denies any associated fever, flank pain or nausea.
There is no past history of urinary tract infection (UTI).
His medical history includes hypercholesterolaemia, hypertension, osteoarthritis, PTSD, and prostate cancer for which he had robotic-assisted laparoscopic radical prostatectomy seven years previously.