Removing dust from the sensor
I just found this post what to do if you find permanent blobs on your pictures in the same area.
Here is the procedure explained from Cy Cheze:
I removed the bottom screws, took off the bottom and left (from behind) side bracket, and then removed the two screws under that panel. I then carefully pulled off the back panel, tugging it gently downward to get free of the upper strip or panel. I then carefully unfastened the control button panel and moved it to the right, being gentle with the connecting strip. I then lifted the LCD panel. Underneath, there is a coin-shaped panel covering the sensor and the lens section underneath. I removed the two screws and lifted the panel. The copper-colored CMOS is "easy" to lift without damage.
The dust particles were very tiny but also very evident and conformed to the pattern seen on zoom pictures. An air blower did not suffice to clean it, so I used a brand new lens cleaning cloth. The specs were stubborn, but did come off without ruining the sensor. Given their color and the rather isolated location, they seem more likely to be of internal origin.
There is no direct exposure to the other side of the lens glass or the lens turrent seams. When seated the sensor fits against a tiny gasket and plastic screen, which fit against the beginning of the lens section. I cleaned them too, just in case, but this can be tricky unless one has a non-scratch tweezers and immaculate settings. I then reassembled everything, which proved to be easier than I feared. The screw slots were no trouble to align.
Result: No more dust spots, so far. Honestly. I cross my fingers and hope not to see any new ones too soon. The cleaning job turns out to be slightly less challenging than fixing a leaky faucet, but with the proviso that the parts are much more tiny and delicate. The other hazard is if you introduce more dirt or have improper tools. A single finger print or scratch would be a problem.