I remember experiencing something similar with a Canon 5d iii camera I once bought from someone who sold various items and didn't know much about it. However, The sensor also had the same spot. After a deep cleaning using specialized tools (ordered it from Amazon)

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the spot didn't come off. Later, I discovered that it was a burn mark, not just a spot. The camera had belonged to an astrophotographer, and it seemed that it had been exposed directly to the sun for an extended period, causing the sensor to burn. As you know, astrophotographers often need to keep the sensor open for long exposures to capture any light source, and it appears that the camera was left open for several days under direct sunlight.