What could have caused this patch?

11 days ago

I have just finished painting my bedroom a very dark blue, and after the final cost I noticed a few patches. At first I thought the paint was just seeing funny, but 24 hours later they're still there. I didn't notice them with previous coats (maybe because the paint was quite patchy anyway) and there was nothing obvious on the wall before I started (house is only 18 months old). There is just a void on the other side of the wall over the stairwell. Could it have something to do with a product used to patch an imperfection on the gyprock (there's another large one that runs down the corner of this wall)? Any suggestions on how to deal with it?
Thanks in advance!