I would like know a fix too. I have 2 maps with that problem on one or two walls. One I managed to cover it up with a few props but the other isn't so easy to cover up.
For me it's neither. When I zoom in to look at it with a sniper rifle I can see props and brushes that are behind it or in the distance. Odd that it's in the middle of a solid wall. Next I'm just going to remake and texture it and see if that will fix it.
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