After Casey gets out of the tub, during the following few minutes, her clothes change from the black slip to her black bra and panties and back again.
A credit card cannot be used to open a framed door that opens in the opposite direction of the person trying to break in. The frame prevents the credit card from being inserted between the door and the lock plate.
At 1:06 into the movie, as a supposedly naked Jill crawls over the passenger seat to close the car door, one can clearly see that she's wearing panties.