And even that might not prove anything... he could have given her a goodnight hug and a strand of her hair could have attached to his shirt... air conditioning could have blown it to another location in the vehicle...
Heck, defense could argue that the way they bumped into each other in the parking garage footage could have been enough to transfer a hair.
If there was no "crime scene" within the vehicle, it would be nearly impossible to make a case on that.
On the bright side (I can't believe I am calling this hypothesis "the bright side"!) if something violent or messy did occur in that vehicle, no amount of car-detailing can eliminate all traces of it.
Either there WOULD be traced of it to be found, or a pronounced absence of anything and a clear indication of a recent and thorough detailing would lead to questions, receipts, a trip to the location, and a forensic search of the equipment at said detailing location...