The only time I see something like this is when the file is open in Sigil and elsewhere, such as in my case, the Sony Reader Library. It doesn't say the file is use elsewhere, but it probably should. It is a work in progress, so I assume that will happen in due course. It seems like there might be a problem occasionally manipulating images.
But these do NOT happen in the background, it happens while you are using Sigil. Is the Dr. Watson program of any use in this case?