I don't even have to look at the pictures: I've seen those spots on plastic containers since my early 20s but didn't know what they were until maybe my 40s, and yeah, they're from the microwave (I microwaved so many leftover lunches from my 20s through late 30s I don't know how I'm not a walking chemical factory at this point; in fact, I probably am).

Anyhow, I don't microwave food in plastic containers anymore but OP does. When I see the spots afterward I just throw the container out as that's even worse than the pre-nuked plastic, in terms of safety. (Dishwashers pull a similar number; last I read on it nothing plastic is supposed to go in the dishwasher, anymore - or should never have gone in it to begin with, I guess.)