Diabetes Mellitus: Given the mention of diabetes as a potential cause, it's reasonable to consider this as the most likely diagnosis, especially since the symptom of occasional frequency is not bothersome to the patient. Diabetes can cause frequent urination due to the high glucose levels in the blood, which increase urine production.

Urinary Tract Infection (UTI): Although not directly mentioned, UTIs are a common cause of urinary frequency and could be considered, especially if the patient has other symptoms like dysuria or urgency.
Benign Prostatic Hyperplasia (BPH): In male patients, BPH is a common condition that can cause urinary frequency, among other symptoms like nocturia and weak stream.
Overactive Bladder: This condition is characterized by a sudden urge to urinate, which could be mistaken for frequency. It might not be directly related to the mentioned causes but is a plausible explanation for the symptom.

Bladder Tumor Recurrence: As mentioned in the question, this is a critical diagnosis not to miss. Bladder tumors can cause a variety of urinary symptoms, including frequency, and given the patient's history, it's essential to rule out recurrence.
Interstitial Cystitis: A condition that causes bladder pressure, bladder pain, and sometimes pelvic pain. The symptoms can mimic those of a UTI or overactive bladder but require different management.

Neurogenic Bladder: A condition where a person has trouble with bladder control due to a brain, spinal cord, or nerve problem. This could potentially cause urinary frequency among other symptoms.
Bladder Cancer (de novo): Although the question hints at a history of bladder tumor, it's also possible for new cases to present with similar symptoms, making it a rare but important consideration in the differential diagnosis.