If the card was exposed during the pitch (flying off the table, landing on the floor - face up or down is considered exposed), then as others have already stated - it is replaced with the burn card,
If this card somehow lands on the floor in the middle of the hand (by the player somehow dropping it), then that is different. The players hand is not killed, but I do not see any reason to expose this card. If anyone saw it, then they saw it, and technically they have an ethical obligation to speak up and say so. I do not think we need to force this player to expose that card for the remainder of the hand.
In regards to straddles - definitely not happening in tournaments. But in a cash game, they are considered a 3rd blind. Which means they have last action. Even if no one has raised, they still have the option to raise, just as the BB would have when there is no straddle and they have last action. As Dave said, that is the entire point of a straddle