I don't know how that got there
You lie about not knowing how it got there because you're the one who caused it and you're too proud to say you messed up.
"I don't know how that got there", after spilling soda on the teacher's desk.
"I don't know how that got there", while wearing a sock on your head and eating pizza from the floor.
"I don't know how that got there", after making a mess in the kitchen and not cleaning it up.