What, exactly do you mean by "in a confined cubicle"? What does that have to do with his measuring his motion relative to another system? What instruments does he have to measure with?A person is in a confined cubicle - say 1m x 1m x 1m. Assuming he is not accelerating, he should not be able to tell if he is moving with respect to any other non-acclerating coordinate system. Are there any limits on his relative speed?