Maybe George, the ex-policeman, who cleaned out the car, told her to clean the knife and pants. or maybe George put the pants in laundry to be washed and knife in dishwasher to be cleaned and Cindy, who behaves like a dependent person or victim, innocently cleaned them or was told to and followed the order. We have all seen George's temper on the TV. It is my opinion that George's angry outbursts are not exclusive to this particular situation or the cameras.
This is what I've thought for a long time. I think GA knew from the minute that trunk was opened that he was smelling decomp. And when he got home he told Cindy she needed to wash them. I also think it's totally in line with Cindy's personality to go ahead and do that. I think Cindy was accustomed to going around and cleaning up Casey's "messes" and taking care of everything.