Internal lens dust—especially near the front element—almost never shows up as a dark spot in photos. If you only see the spot in some images, the more likely cause is dust on the sensor, not dust inside the lens.

Why it appears inconsistently: sensor dust is most visible at smaller apertures (higher f-numbers) and against plain backgrounds, so you may not notice it in every photo. Dust or marks on the rear element can have more effect than front-element dust, but usually simple external cleaning is enough.

What to do:

• Check and clean the sensor first using the camera’s sensor-cleaning cycle.
• If the spot remains, inspect for sensor dust and consider a careful manual sensor cleaning.
• Clean the rear lens element if needed with proper lens-cleaning tools.
• Don’t worry much about a little dust inside the lens unless image quality is otherwise affected.

If your camera supports it, Canon’s Dust Delete Data feature can also help reduce dust spots in processing.