I don't know how that got there
You pretend you don't know how it got there because you were the one who put it there and you're too lazy to admit it.
"I don't know how that got there", while eating the last slice of pizza from the floor.
"I don't know how that got there", after knocking over a whole shelf of cups.
"I don't know how that got there", while wearing a sock on your head.