Bast would have been pretty sure that all the dirtiness was all on Penny's part. Doing that on purpose was at least third date behavior if not later. This was all just a bit of a test to see how things went when they weren't meeting up in the library or passing notes across the desk that eventually got burned so no one else could read them later and know what the two were exchanging glances over. Then considering his flirting needed work even the dirty third date antics might not go all that well, or sound really awkward. Apparently he needed to stop trying so hard.
He tried just maintaining innocence on how he was affecting her and how she was turning all red. "Well I'll still try to be careful about anything that could make you hyperventilate." Things were starting to look clear enough though and he was fairly sure there wouldn't be any more disappearing acts enough to take the car out of park and start getting them out of there. They could still talk along the way after all.