It looks like a light leak, incomplete joint between the flooring and wall. 3dsmax also does this. I recently had an issue on an interior rendering and Micha suggested putting my model in a large enclosed "box". It worked and eliminated my issue. If there is no exterior light reaching this portion of your model you could try that, otherwise the joint between the 2 will need to be examined. It does look almost like a reflection though, you are right.
Hello! Putting a huge box around the model sorted it out yes, the light spots are gone. However, it is not really a solution for me because I am rendering a video that moves outside and inside the whole time. But at least it is something I could possibly work around if I don't manage to fix it any other way - thank you very much that was really helpful!