```ts
/**
 * Returns the minutes between the first and last on which the monitor did not run.
 * @param minutes - An array of minutes in increasing order when the monitor ran.
 * @returns An array of minutes in increasing order where the monitor did not run.
 */
export function pollGaps(minutes: number[]): number[] {
  if (minutes.length < 2) return [];

  const gaps: number[] = [];
  for (let i = 1; i < minutes.length; i++) {
    if (minutes[i] - minutes[i - 1] > 1) {
      gaps.push(...Array.from({ length: minutes[i] - minutes[i - 1] - 1 }, (_, j) => minutes[i - 1] + j + 1));
    }
  }

  return gaps;
}
```