This diffuse pattern appears in all of my recent photos. I don't know what it can be. The camera is a Canon.
There seems to be something on your camera's sensor. If you have saved a "dust delete data" image and have it enabled, in camera JPEGs and previews (such as those attached to raw files) will attempt to filter it out.
When you first open a raw image in Lightroom in the preview module, you're almost certainly looking at the in-camera generated JPEG preview image that is attached to the raw file. Once you begin editing the image, Lightroom will render a new interpretation based on the contents of the raw data, rather than the JPEG preview you were initially seeing. At this point the Canon dust delete information will not be used by Adobe Lightroom and you will see the artifact.