Maybe you had lotion on your hands before you left and forgot to lock your door. That's kind of the only thing i can think of since i don't live with you and there most probably isn't any greased up burglars running around in your neighborhood.
And you didn't know which category it went under so the first thing you chose was math?
Well...... It could be forgetfullness on locking a door. Maybe your mom was cooking and she had oil penetrated in her hands, which then reached the door knob. There are many different possibilities, but this question is a little vague. This may be something you have to think about since you are living in your house.
