Whenever i have seen anything like that on an LCD screen it has been caused by hotspot.
I have noticed mine has the same thing but i have to look very, very hard to see it
I had never noticed this before, but knowing where to look and looking very carefully while having a completely even white background I can spot the spot on my phone as well. Not something that bothers me even now that I know it's there as in my case it's undetectable in normal use, but I'm curious for any further info as well.
Same issue :(