It was most likely something on or very near the front of the lens, not a sensor problem. Because it appeared in only one frame and then disappeared, a temporary cause like a water droplet or even a small insect on the lens is more likely than dust on the sensor.

Sensor dust usually shows up more consistently across multiple images, especially when photographing a bright plain area at a small aperture. A quick test is to photograph an evenly lit bright surface while deliberately defocusing; if you repeatedly see the same spot in the same place, that points to sensor dust.

Also inspect the front element of the lens for smudges, drops, or debris. Keep the lens clean with a blower and proper lens cloth, and be cautious with sensor cleaning since it can be damaged if cleaned improperly.