That's not exactly what I expected to see. It looks like there could be veining, which means it's at incubation temperatures. If it wasn't for what looks like veining I'd have said a blood spot. But what looks like veining seems to be too tight. It should be more spread out. It just doesn't look right for development.
What could be out there to provide that kind of heat source? You should not get that much development if you leave them out there for no longer than overnight even under a broody hen. And you said it is becoming more frequent. If that were development I would expect it to be really rare.
My thought is a meat spot but I don't know what is going on. It does not look right for development, a blood spot, or a meat spot. The timing doesn't sound right. If you are in the US you might try contacting your county extension office. Not all extension offices are the same but a good one should contact a chicken expert. It would be really interesting to see what they have to say.