I don't know how that got there
You say you don't know how it got there because you're the one who put it there and you're too tired to explain it.
"I don't know how that got there", after making a mess in the kitchen and not cleaning it up.
"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 the trash can.