Lighting build issues - please help!

So after building the lighting in my scene (which is pretty poorly optimised, I am somewhat of a newbie to unreal), I encountered these weird splodges of light, which have no source as far as I know. Every time I build the lighting, they appear, but not always in the same place. Could it have something to do with volumetric lighting? Or is it just because of next to no optimisation? Any help would be greatly appreciated!