What's this dark spot in the image?

Presume you have a record of the shutter count of the 6D and the file number of the image is the next number along..if it isn't so you can ask for the first image. The customer might've removed the lens to fit another, thus allowing particles in, and as another has said no matter how fastidious you can be cleaning the sensor the mirror box isn't a 'sterile' area...
 
I've blurry spots like that appear and disappear between shots (which i can even post) which look exactly like that at f1.4

it has to be something environmental, whether an insect or bit of debris in the air.
 
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