Perform at your own risk, i’m just sharing how i fixed my problem
As i was randomly snapping pics tonight, i noticed there was a dark spot in the photos, and it appeared in all the shots in exactly the same spot.
Notice in the top right corner. See below for magnified crop.
Initially, i thought it was a hot pixel. That occurs sometimes in Canon EOS cameras and is easily cleared by using this technique: manual sensor cleaning mode, 60 seconds, then power off, power on. So i gave that a try, twice, and it didn’t work. The dark spot was still there. That’s when i decided that it was probably a physical item, possibly a speck of dust on the sensor. A hot pixel is generally also blue or red or purple, not black like the one i saw. Black tends to mean dust or some physical element on the sensor.
Don’t try this unless you’re very sure you know what you’re doing. So i went back to Manual Sensor cleaning mode, accessible from the menu system. What this does is raise the shutter permanently for as long as you leave the power on, thus exposing the sensor behind. With the sensor thus exposed, you can reach it with a cleaning element such as a clean air blower, or a dry cotton swab, or like i used, one of those dust pens (like the one pictured below). By using a small penlight, i spotted the small speck of dust immediately on the sensor in the corner of the sensor, gave it a quick swipe with the dust pen to clean it, then quickly powered off the camera to bring the shutter back down.
And that was all it took! Sensor is clean again, and no more dark spot in the frame.
It’s probably a good idea to do this in a clean and dry room where there isn’t any dust floating around in the air. The sensor will be exposed for 30-60 seconds at least, and it’s very easy to make things worse than they are unless you’re careful.