AFAIK photovoltaic cells don't produce heat, they produce electricity. You suspect if they're not producing electricity they retain more solar heat? I'm having trouble wrapping my head around that.When the battery is fully charged, the current drops to zero and thus the batteries are consuming no power. The panels also produce no power when the current is zero, so nothing is wasted. The panels have potential to produce power.
I have long suspected (but never bothered to test) that panels producing power should be cooler than identical panels in the same solar situation that have nothing connected to them. If that is the case, then the answer to your question is that unused potential simply heats the panels. Somebody with an IR imaging system could test this easily by putting two similar panels next to each other in full sun, short one of them, leave the other open, then see if they are at different temperatures.
There are thermal solar panels, but I think those are a different animal altogether.